I would have to go with black ops2 just because their zombie field was so much better then modern warfare 3. They changed to much of the game style and the interface when they came out with modern warfare 3 and black ops 2 was definitely the better choice if you plan to do a LAN party or even play online with others. In Modern Warfare 3 they took to many elements of first person shooter out that belonged and is in Black Ops 2.
While both games are great, they're very similar in game play and both offer challenges online and off, Modern Warfare 3 offers much better online game play than Black Ops 2. The match making in Modern Warfare had more variety of game types than Black Ops. I usually buy a game with the plans of playing online with friends so Modern Warfare won me over there. Their off line game play is also just as great if not better than Black Ops 2 also.