I've been to both the Mandalay Bay lazy river and just recently the MGM lazy river. I have to give the edge to the MGM. It's a lot longer, plus you can chill in it with your drink. This can be a plus and a minus depending on how many drunk fools are spilling their drinks in it. The Mandalay Bay lifeguards will hunt you down if you try to bring a drink in there.
The other benefit of the MGM lazy river is the music coming from next door at Wet Republic. Big plus there.
I got there at around 3:30pm and found out that the river closes at 6pm which kind of sucked but we ended up going over to one of their other pools which stayed open till 8pm.
Also, want to give a shout out to MGM staff that found my ATM card in the river the next morning and turned it into security.
Big negative, price of drinks :-(