I think it just depends on what you want for "fun"?
Like your friend said, its the difference between a gritty, great game with a serious tone, and serious production values, and a more light-hearted sandbox game.
Fortunately for you, you have Saints Row and GTAIV, so you get both arcade-y sandbox action (SR) and more "mature" sandboxing in (GTAIV). And throw Crackdown in the mix, and you got sandbox heaven!
So if your wanting a Sandbox game, it's all on the 360.
Back from the dead, I'm afraid.