I prefer Saint's Row 2.
That being said, I also really enjoyed the original Saint's Row, (it was my favourite sandbox game until the sequel and better than GTA4 IMO), so consequentially GTA4 was a let down in the fun department, once the initial impression or the pretty graphics had worn off.
I never found the "grinding" in between missions to build up my respect to be a chore, because the diversions were a lot of fun for me, and initially I spend most of my time doing them for fun, so I always had enough respect to do the missions.