Medal of Honor is a unrealistic shooter in realistic situations, it has superb sound effects and it is made on the frostbite engine. It is only set in iraq and Afganistane and tells some pretty emotional in depth stories for war. However, as innovation goes, medal of honor does not have much to offer, the weapons and game play are pretty standard, there does not seem to be much new creative awesomeness...
Call of Duty Black ops is a game with great graphics and adequet sound effects, it is made with the unreal engine 3, (arguably) the best gaming engine to date. It is made using the call of duty style of gameplay that you love so much, and if you hated COD:MW2 dont worry! it is made by treyarch and not infinity Ward, it has many new weapons and innovative gametypes... however the average unrealistic in unrealistic situations cod gameplay is still present and there are still using ruthly the same engine and gameplay as all the other COD games from modern warfare...
I don't know, personally i am saving up and trading a few games so i can get BOTH! but if you can't do that... i hope my information helped