Resident evil 5 is a good game if you are looking for a shooter that doesn't take itself that seriously, isn't that scary but manages to create a strong plot as well as gory monsters and fights. It also has a weapon purchase and costume system that lets you customise your character somewhat.
GTA 4 is free roam, got AMAZING reviews , there has been no word of another GTA coming out soon and it has 2 downloadable content packs if you get bored. it has a compelling plotline and at times is humorous with occasional dark humour.
Personally i would recommend buying GTA 4, as it has more content and a much better multiplayer component.