I think both games have aspects which are better than others. For me, I found GTA IV, much more immersive as I felt it had superior character development. For me: Romeo, Niko, Pegerino, Ray, Elizabeta, Dwayne, Brucie, Lil Jacob, Dimitri, Vlad where all far more captivating and memorable than characters like Martin, Lamar and Lester. The only three characters which I would consider interesting on V where the three main protagonists. While I do not think the exploration is better in IV than V; I still prefer IV, because of the story which I have finished three times.
Get IV and if you do not enjoy playing after completing around 30 missions then stop playing. (It will take a substantial amount of time to become immersed.)