Nobody knows whether or not GTA IV is any good on the PS3 except maybe Rockstar since the game hasn't been released yet. It's still about a month or so away.I think the only must have games for the PS3 are Uncharted, Resistance: Fall of Man, and Ratchet & Clank.Heavenly Sword and Motorstorm are nice games to have, but they're not must haves.
We can't really say, since the game isn't out yet. Both versions will have exclusive downloadable content, however, PS3s Blu-ray offers more space and PS3 is more powerful. Time will tell.