I liked them both. I though GTA V had a pretty good story, great gameplay, good characters, it knew how and when to be funny unlike certain games (Saints Row III, too ridiculous for its genre, didn't even play IV after that U_U) And the side missions(marathons excluded...)were good.
I thought AC IV had an uninteresting story, but with some interesting or memorable parts. The characters were good for the most part. The combat was solid, with the usual AC trademark kill animation glitch(Like stabbing someone in the ass instead of the abdomen, but I didn't mind that at all.) The naval combat was the best part of the game. It was just very fun in my opinion