Had an unexpected trip to Vegas for work and on consecutive nights played from 7pm til 3am in each venue. I thought a side by side comparison would be helpful to others. This was my first time playing NLHE in Vegas.

Room Quality:
I thought both rooms were very nice. I would give the edge to the Venetian simply because their room is off to the side and thus quieter. Both have very nice tables, good chairs and after an 8 hour grind in each I did not have a sore butt. As for the tables, that beloved/hated marble racetrack around the edge at MGM was annoying because it is lower than the felt and if you don't catch your cards they flip up a little. I doubt that anyone saw one of my cards as a result but it could happen and it is distracting.
Venetian 5/5 MGM 4.5/5

Competition:
I tend not to move around and stayed at one table for the entire night in both places. Newcomers flow in and out regularly as they bust out or clean up. In general, there were better players at the Venetian and clearly a few new most of the dealers well. At MGM there were more young guys who play loose/aggressive and will shove all in with nothing. In both cases, I finished the night up slightly but I really wasn't bankrolled to go after the loose aggressives so I was pretty nitty. Had I been properly bankrolled I would have made a lot more at MGM. If you hate games where people routinely bet 5x preflop with junk and then push hard the MGM will be painful. If you are patient you should clean up. The super aggressive loose guys at the Venetian get eaten alive more quickly because the regulars take aim, wait for a hand and felt them. Phil Helmuth would have a hissy fit at the MGM over having his early position raise called by 35o and getting rivered by a college kid. Playing Tight/aggressive at the same table for several hours you can measure the general skill of the others. At the Venetian, when I led out with $10 from middle position after an hour or two of 15-20%Vpip it was fold fold fold and I had to loosen up a bit. After folding 20 straight hands at MGM I rereaised a $10 bet in middle position with $25 and got called by two guys who were there with me for two hours, when the flop came A K 3 rainbow and I checkraised all in I got called by a guy with KQ who seemed genuinely shocked to see my pocket aces. Wow. My next reraise after 3 players called a $10 bet from under the gun drew immediate folds and showing the sev/deuce ensured plenty of action when I went back to being a nit.

Venetian: 3.0 MGM 4.0

Dealers:
Not much to complain about in either venue. The dealers at the Venetian were just smoother and more polished over all and were a bit warmer. If I were a betting man (oh wait) my assumption would be that the Venetian offers a better package to their dealers and hence have their pick of the best. A couple dealers at MGM were short and cranky with players. In both cases the dealer was correct but in both cases could have been smoother about making their point with a smile. A few cards hit the floor at MGM, once causing me to toss suited cards I might have played. One dealer error at the Venetian: a guy was hiding his cards and never attempted to act after the flop or turn and dealer missed that he still had cards. After the river it was noticed and the dealer turned briht red and immediately called in management to rule. The player was to my right and I asked him quietly if he'd intended to fold after the flop and he said yes, his hand was declared dead to no resistance and he was politely reminded not to cover his cards and to let the dealer know if his turn was missed. I add all of this detail to point out two things: One, based on the immediate "OH CRAP!" reaction from the dealer and mgt I assume that such errors are very rare and taken very seriously. Two, it could not have been dealt with more professionally.

Venetian 5/5 MGM 3/5

Beverage Service - Quality:
No contest, the Fiji water, Red Bull and Top shelf beverages at Venetian are 5/5. MGM's well drinks, limited supply and cheap selection are not comparable in quality and price. Personally, I didn't find anything wrong with the Rhino (red bull sub) or cheap water at MGM which was all I ordered. Had one real drink at Venetian, asked for Maker's rocks and got Maker's, not well bourbon but I typically do not drink when playing. Saw others ordering cognac etcetera and never heard "we don't have that' at Venetian but heard it all night at MGM.
Venetian: Seriously, 6/5 MGM: 2/5

Beverage Service - Wait staff
Venetian kills here. Their staff is gorgeous and attentive as hell. You don't have to wave or shout for service, they are around often and if they aren't around and you look they will catch your eye and immediately come to take your order. MGM was another story all together. I wanted some water and waited more than 30 minutes. It became a table joke as I was waving and speaking out with "Excuse me, Ma'am????" three times. At that point dealer one (went through more than one dealer while waiting) responded to my "Hey, just curious, can you guys see me and hear me OK?" with "It isn't personal, they pretend not to see you until they are ready to take your order. Finally the guy across from me politely stood, touched the girl's arm, pointed to me and said "the gentleman has been trying to get your attention for a half hour". She told him "it hasn't been that long" without acknowledging me. At that point, the dealer (dealer 2) apologized, said that he thought it was very rude of her not to at least acknowledge me and say "I'll be right back" and just walk off. He presssed his button for management who was there a couple minutes later and explained, the manager asked what I wanted and a minute later the waitress showed up with my water, I thanked her and tipped the dealer to the chuckles of the table and the chagrin of the waitress. She was probably attractive 20 years ago (no kidding) and seemed annoyed that I dared to complain. I apologized to the table and said "sorry guys, not trying to be a dick, i am just thirsty" ...all agreed that I was not the problem. I add all of this detail to make a point as I am sure that MGM's management reads these posts. When you do not treat people with minimal courtesy they recall it vividly when talking to a few thousand of your potential clients. Worse, the customer may be capable of a rational recollection of the events and provide enough detail to make it clear that he is not prone to bitching.

Venetian Looks 5/5, Attitude 5/5, Attentiveness 5/5
MGM: Looks 2/5, Attitude 2/5 (Server covering our table was replaced with a nicer and more attentive one when grouchy granny was pointed out) Attentiveness 2/5.
Side note: Pay for performance trumps pay for time in union at the tip of the sword in the service industry, period.

Management:
Both use the tableside system to swipe your card and both found me a seat immediately. I measure management in a few ways: How well does the floor staff reflect management's insistence on customer service? Venetian 5/5 MGM 3/5.
How does management convey their customer service attitude when called to the table personally? Both venues 5/5
How well staffed are they overall?
Venetian 5/5 - it isn't just the bev servers, at the Venetian the guys to my left and right ordered food and I suppose my growling stomach was obvious, the dealer asked if I wanted a menu too, caught someone's eye, nodded toward me and a server appeared, took my order, brought my dinner to the table in 5 minutes. Also, when the dealer asked for my player's card and I did not have one they asked if I wanted one and had a runner take care of it instantly. At the Venetian you just feel like they are there to cater to you, the service is effusive without being distracting as you would find in a good restaurant where the staff is so well choreographed that every need is handled before you notice it. The MGM is good, not great. I am guessing they spend a lot less on support staff. In fairness, if you never went to the Venetian, MGM would seem good. MGM gets 3/5. Whomever is in overall charge of the Venetian's room should be the highest paid poker manager in the world because they nail every detail and you can just tell that any error is an embarassment.

Final thoughts and impressions:

In both games I had a nice time. The players at the MGM were more fun to chat with and we had a very friendly game where winners were acknowledged politely and with good humor. At the Venetian fewer guys were drinking, fewer guys were stupid and players were generally more serious and reserved. In both cases I bought in for $200, played tight/aggressive and made a whopping $100 or so after 8 hours but I was there to have fun and relax while getting a feel for playing in Vegas and measuring my skills. I'm an online player who generally plays 4 tables at .50/$1 simultaneously and I do a couple local home games so I'm not a veteran live player and have a lot to learn about reading people in live games. This trip was unplanned and normally I'd like a bankroll of $2500 for playing $1/2 rather than a few hundred bucks but I did not have time to take cash out of my online accounts for the trip. Even with tips and rake I came out ahead after 8 hours in each venue so for the decent recreational player you can go to the top rooms, play tight and play position and expect to do well enough to play all night without getting hurt. I'd recommend a night or two of playing small bankroll before deciding to play for keeps if your background in poker is like mine.
Hats off to the Venetian, they really run a great room.