I've heard people call Saints Row 2 the true spiritual successor to GTA:SA, and I kind of agree. I've played GTA4 and SR2 and as far as plain ol' fun factor goes, SR2 is that some sort of zany over-the-top craziness that GTA:SA has.
Despite both being suburban/big city sandbox games, GTA4 and SR2 are two very different games and are both good in their own way, so it's hard to pick one over the other. I'd definitely recommend both to anyone who likes suburban sandbox games. I guess it comes down to this: Do you want a more serious game that offers commentary on American city life and society, or a game that's more whacky and fun?