Saints Row 2's superior combat and flat out more enjoyable gameplay give it the edge over GTA4.
SR2 doesn't reinvent itself, but it is damn fun. The combat was already vastly superior to the GTA series with the first title, so not much needed to change this time around. Character customization is also another great feature that you can get lost in for quite a while.
The overall attitude is more light-hearted/less-serious than GTA, and it only adds to the enjoyment of the game. These titles are asinine to begin with, it doesn't help when the storyline is ultra-serious.
If you have the first game but didn't think too much of it, you won't necessarily be wowed with the change from the first game to the second. However, if you LIKED the first game, there is zero reason not to pick this one up.