A well made slapstick parody version of GTA IV
GTA IV is in my opinion a close to realism "crime-simulator", while as Saint's Row 2 is more like an "arcade-crime" type of game.
The reason I put it like that is to compare the different things in these two games:
1. Controls
* GTA IV car handling is smooth and has realistic physics imbued.
* SR2 car handling is more flaccid and loose.
* GTA IV main character Niko moves like a drunk on drugs (even for a "realistic" crime simulator, I for sure haven't seen people move like him, unless they were drunk...or on drugs...or both).
* SR2 main character moves just like CJ from GTA SA.
* GTA IV Niko fights as if he had been trained by a 1920's fisticuffs champion.
* SR2 character fights in a quite realistic fashion (weirdly enough!).
2. Graphics
Well, this is where GTA IV beats SR2 with a few steps.
*Liberty City and everything in it is very detailed (even though it can be improved further, but still..) from the high sky down to the pavement.
* Stillwater in SR2 looks rather dull where surroundings and whatnot is quite boring to look at. Can get a bit tedious when played for many hours on end.
3. Gameplay
This is where the games divide pretty strongly. Whereas GTA IV focuses on the story and main missions, the smaller missions don't tend to fuel the playing hunger much, considering going for the pigeon hunt (Flying Rats) is not much to hang up on. Sure you can do racing, but that's where all the fun ends.
SR2 on the other hand offers a bit more than just the main story line - you can choose from variety of stuff to do (flashing, streaking, poker, blackjack, racing, mugging, and much more)- to mention a few. Fun for the whole gun!
Summary:
If you want realistic crime simulation with little effort and more or less no fun in it for you, then choose GTA IV - but, if you prefer non-stop brainless fun, and non-realistic environment, then Saint's Row 2 is for you.
The choice is entirely up to you.