Should I buy grand theft auto 4 or L.A. Noire?

I want to buy either gta 4 or LA Noire. I've played through most of gta 4 with my cousin so im leaning towards la noire. But in gta 4 i remember that i could just go around the city and do whatever and postpone the missions. Can you do that in la noire or is it all story mode? In la noire can you just drive around causing chaos?