I've always liked the Infinity Ward games better. If you know anything about the history of the development of Call of Duty, you will know that Infinity Ward is the real game developers and Treyarch just copy and pastes their work and polishes it up.
World at War is just an exact copy of COD4, there is even code for flashbangs from COD4 in the WaW game files and a model of Captain Price's head.
Notice the file path at the top of the window. Every Treyarch game is just a polished version of the previous Infinity Ward game.
I feel both series have gotten progressively worse though. Especially the Treyarch games. I can't remember ever having fun playing Black Ops 2. The lag is just terrible and the game was focused on competitive gameplay and lost it's fun. The zombies are also boring. I've made box maps more fun than the BO2 zombie maps. Black Ops 1 I only like the zombies and sniping.
COD4 still remains the best COD game made, although you have to play it on PC due to all the script kiddies and their so called "mods". MW2 was a good game until the trickshot clans took over. I still greatly enjoy MW3 and I'm ready for the next two years playing Ghosts.