Hello again, These I have both of and have beaten both. Both are execellent, same kinda answer as with farcry. But if you only have Black ops, get Ac3 for a 3rd person game with pretty good multiplayer. In addition to having multiplayer, that Dishonored does not, Ac3 is also a longer game if you get all the side stuff too. Dishonored is half as long, unless you play the game through to get both storylines/endings.
AC3 is way longer and has multiplayer (which is okay, not too addicting), dishonored is good but not open world and you can beat it in 10 hours. If you do all the stuff in AC3 in SP mode it'll take 35-45 hrs