Which one is better?
Since they are both on sale on steam.

Watched a Let's Play of XCom, It looked like a pretty fun turn based rts except the tutorial did feel a bit like it's holding your hands. In general though it looks pretty fun.

As for Darksiders 2 I haven't seen much gameplay but it did look a bit fun, and saw some good reviews

Dark Siders 2 and this isn't off-topic

League of Legends?

I bought Awesomenauts for lulz.

Dark Souls

X-Com is a flawed but fun game and its nice if you like TBS games and are up for a challenge. Normal is reasonably difficult on your first run though but pretty easy after you get the hang of the game. 'Classic' difficulty is brutal and I don't even want to look at Impossible. On top of that, in Ironman mode you can't load old saves if you make a mistake.

I've never played DS2.

X-Com. Haven't played it but heard its good, and Darksiders 2 was massively disappointing.

X-com if you want like a strat game and darksiders for button masher i suppose.

Xcom hands down. Get darksiders 1 the story and gameplay are as equally lack luster as the second just 30 dollars cheaper