Having played these games it really does depend on what you are looking for at the moment.
X-com is a turn based tactical shooter and base building strategy game
While I like it and its presented very nicely, unfortunately I find it lacks depth in several areas this I measure in comparison to the originals and the various X-com clones (UFO; aftermath, aftershock, afterlife. UFO: extraterrestrials, UFO: Alien Invasion) I really feel that they could have, and should have done more with this game than they delivered. Perhaps this will be fixed in DLC, or the various mods which attempt to rebalanced and improve the gameplay. That having been said if you feel up for a squad turn based tactical shooting game, with strategy elements this is the one.
Dishonoured is a stealth-em-up melee super assassin with magical stuff. Basically looking glass's Thief: the dark project and Thief 2: The metal age with more emphasis on assassination and a more modern and prettier engine.
I put my vote in this game, I found it fun, a completely new IP, it had an interesting story, and it's one of those games you can complete in multiple different ways, from sneaking and no combat, to knocking people out, or inhabiting them, or setting traps, overriding turrets and killing enemies environmentally, or going into a killing frenzy with swords or guns. It's also one of those games which you can breeze through faster, or take more time over depending on how much of a challenge you want to set yourself, and how much exploration of the environment you wish to perform, and you get rewarded for doing so, both game rewards and flavour.
I really think if you want a new IP dishonoured is the way to go. Especially if you can only play one and you have no other preferences. If you can, pick up both, since they are very different and will appeal to different tastes. There you go, a bit rambling but those are my thoughts.
What are you hungry for at the moment?
Edit: added a comma and two spaces.