I am a huge GTA fan and own all the games but Saints Row 2 is easily the best of the sandbox series. Saints Row 2 is full drop in drop out co-op campaign with everything that was missing from the GTA4 campaign. You can see your co-op partners character, buy clothes, jewelry, completely customize your character top to bottom including walk and personality, buy cars from car dealerships, buy bikes from bike dealerships, buy houses just like in Vice City and San Andreas then make money off of them, buy every single store you see in the game, buy boats and boat docks, customize your house, parachute from anywhere in the game, fly planes, join fight clubs, drive monster trucks, quads, dirt bikes, car surf, fly a helicopter, customize your homies and the way their cars look, customize EVERY vehicle in the game (if you can drive it you can customize it), and much much more all with a co-op partner. Every mission every level everything is all co-op. You can hold people as human shields just like in GOW2. Pick up any object in the game and throw it or use it as a weapon. If they were going for realism in GTA4, they fail because realisme would allow me to at least buy a car or a house.
Seriously Saints Row 2 campaign crushes GTA4 campaign by a lonshot. But one thing I will say for sure, the multiplayer in GTA4 is much better than the multiplayer in GTA4.