Before reading below, you might want to vote - I'm being very biased, since I've already made up my mind.
But if you want to see what I think and why, go ahead and read.
If you vote post why you voted!
Played GTA 4 quite a bit and frankly... not very impressed.
Huge fan of the series, played the original GTA (the top down 2D one, not GTA3 - which is what most people think is the 'first GTA' for some reason) all the way through San Andreas.
I think that the game built upon your freedom and property control until San Andreas, which included some awesome features like gang territories, muscle, weight, car mods etc.
Then, we have GTA 4. Yes, it is beautiful. It LOOKS great. The story is pretty good. The cars are gorgeous and handle beautifully. But it seems like they've removed all the awesome features from San Andreas, and even some of the earlier ones.
Saint's Row 2
Now, after I played GTA 4, I tried out Saint's Row 2. I wasn't expecting a whole lot, never having played Saint's Row before, and was thinking it'd just be a cheap GTA knockoff.
But damn! The freedom is incredible. I have gang territories again, and I need to defend them. I have a wardrobe that extends a mile, a ton of tattoos, crazy haircuts, and I can even customize my race/sex/voice. I can mod cars again, even more options than in GTA:SA! Not only that - but I can SAVE those modded cars, and a lot of them. I can even get them delivered to me, and they'll return to my garage if they get destroyed.
I can customize my gang. Their styles, their hand-signs, their clothing, even our gang tags. Even the cars my gang will drive.
SR2 > GTA4. Yeah, GTA4 has SR2 beat pretty squarely in the 'ooh pretty' factor. But the 'ooh pretty' factor only lasts a couple hours, or days at best. The gameplay is stale, there's really very limited freedom (even compared to earlier titles in the same franchise!)
I've always been a fan of games that had real freedom in them. (Starflight, Elite, Wing Commander: Privateer) So, GTA4 was a giant step BACKWARDS in my book. SR2 is what GTA4 should have been.