[HEADING=3]Spoiler-free view:[/HEADING]
I own both and both as equally good as each other. I've probably spent just as much time playing both too, and they both have a pretty decent amount of replay value, though Dishonored has just a little bit more.
Dishonored plays quite uniquely. Some describe it as first person Assassin's Creed. It's not.
The combat is very similar to Skyrim, clunky and something you want to avoid, but movement is very similar to Mirrors Edge, with fluid parkour. The stealth aspects work out really well too, like Thief and Splinter Cell, and the supernatural powers are what gives the game its identity. Very good game indeed. Despite having done nearly 3 very different playthroughs I'm still finding really abstract yet perfectly functional ways to beat missions.
My only Criticism would be the combat. full-out fighting is not its strong point and the inventory system is a bit lacklustre. Everything you pick up transmutes into coins. Also a small-medium chunk of the game stops offering you consistent multiple paths, and instead turns very linear, but it clears up after those 2-3 areas are over. This only happens for about 2-3 hours, and multiple paths DO still crop up within it, but unlike the rest of the game before and after that section, they aren't consistently there.
Xcom, however, almost feels like a rogue-like. It's not, but it is a very, very good turn-based tactical strategy.
It's a very challenging, very strategic game. It took 3 goes for me to finally win on Normal, and I gave up on Classic. I've sunk tens of hours into it already and it's simply a brilliant turn-based game. The base management is arguably more important than the missions, but without successful missions you won't get the funding for your base. A very intuitive, intricate and clever game where a single playthrough will take up to 100 hours to complete, and then there's no guarantee you'll win a second playthrough just as well, if at all. A very clever and challenging game. It has been excellently streamlined to appeal to modern gamer expectations (maybe a little too far), and it plays superbly, looks great, loads fast and is highly dynamic.
My criticism with Xcom would be the Random Number Generator in harder difficulties, as it seems to be biased in favour of the Aliens. In normal, you'll most likely hit a 50% on the first or second (or if you're unlucky, third) shot, as the law of averages states you should. HOWEVER, in harder difficulties you'll see yourself missing desperate 75% chance shots far more often. It wasn't even until my harder post-completion playthrough I thought you could actually miss a 39% shot consecutively by 3 people across 5 turns. This happens regularly in Classic and Impossible difficulties. Not to mention everything's more expensive, you get less council money, your guys are weaker and less accurate anyway and the aliens are stronger hitting and more accurate. The bugs are a big problem too - both them and the RNG have cost me an Ironman Classic (no-reloads, hard) playthrough. Also, consider yourself lucky if the multiplayer doesn't bug up in your very first match. Watch Total Biscuit vs Angry Joe round 3 of Xcom [http://www.youtube.com/watch?v=hZ9QHRWB6p4] for a multiplayer verdict. They were playing on the patch release that's out now.
[hr]
Both are worth their reputation's weight and both excel in their represented genres. they both have atmosphere but Xcom is a little buggy. Dishonored has better replay value, but Xcom has better lifelong appeal (it won't start collecting dust, you can always boot it up and get a refreshed experience). If you like longevity and to use your brain more, go for Xcom. If you dislike luck elements or prefer a good storyline, go with Dishonored. If I HAD to choose, I'd say Dishonored, but only because Xcom is still a little buggy. It's still a really hard choice to make.