Dishonored or Assassin's Creed 2?

Let me start with the fact that I love both AC series and Dishonored and completed all AC and DH as well :>

AC2 is good but not so great, it's nothing compared to next one (Brotherhood). If gameplay is what you're looking for choose Dishonored. It may not be actual oldschool stealth game, but at least it gives you some kind of challenge. AC series tends to have good story, lot of side missions, but generally games play themselves - they give you no challenge at all, except from rare forced hard missions. Main pro behind AC series is story and lore and easy fun free-running. Dishonored, while not having such a deep story and lore (well it's just 1 game vs big series ;p), it offers fun game mechanics.

So if you're looking like you're saying for gameplay - choose Dishonored :> Also to really appreciate AC you have to play more than just one game ;p

Definitely Dishonored! Stealth is satisfying, there are lots of options and you'll roam around feeling like a badass the whole time. The aesthetic/art style is gorgeous too. AC2 is fun with lots of freedom but gets feeling repetitive. Great game in its own right but in this case I would go with the D ;)

I've played both games and they're awsome. but if you want a challenge then go for dishonored.
another thing is that if you play AC 2 you'll definetly want to play the other games as well. but if you buy dishonored (not the goty edition) it's ok. you will not necessary want to play the dlc