GTA V because I've never owned an XBox and watching the 24 hour Halo marathon (currently halfway through part 7) has me convinced that I was truly underwhelmed by the series as a kid and wasn't just blinded by PS2 vs. XBox fanboyism.
Then again I also think FPS games are generally bland; the last times I can remember enjoying FPS games was Rare's N64 games, Turok 2, Call of Duty 2 on PC and Medal of Honor Underground on PS1.
Why would you even want to go with Halo 4? It's probably going to have paid reviews and only last 2 months (if even) then feel outdated compared to most modern games and you'll get bored. Grand Theft Auto 5 however, has a really good track record and will last you a while with the replayability.
I say go with Grand Theft Auto cause Halo has proven itself not to be a great franchise to buy into and bad at lasting appeal. I've always felt there were better shooters than Halo, but it got a lot of attention because it was the only thing on xbox.