I think there's a lot more variety in the missions in GTA. The missions in NMH were only unique when you got to the boss fight. Everything else in the game was pretty mundane.
My only gripe with the original commenter is that he said there is 30-40 hours of "backtracking" in GTA, which is a major exageration. NMH has a far larger percentage of wasted time which was the entire open world and most of the missions. The only fun part of each mission was the boss fight! They were so good that it keeps you playing, but I'm pretty sure most people will play GTA enough to experience some decent missions if not the same kind of boss fights.
I'm not trying to convince anyone to like GTA. I've never liked GTA. The combat sucks. Supposedly in GTA IV that has been greatly improved. The main point I'm trying to make is that the open world of GTA isn't a negative compared to a game ike NMH, and it certainly doesn't require 30-40 of "back tracking."