I know you don't want to hear the "you can't go wrong with any of those" answer, but I can speak personally for Sleeping Dogs and XCom at least. Totally depends on the type of game you want to play, not really on how many hours you'll get out of any of them. Sleeping Dogs is fairly lengthy, especially if you like the open-world collection aspect of getting and unlocking every little thing. XCom can last you as long as it takes you to get bored with the mechanics.
I'm not deep into XCom, but man do I personally love tactical games, and it's been a blast so far, so if you need an answer besides what appeals to you, get XCom.
When you're done with the story I feel Dishonored is spent, it never felt special to me, or different. I feel it is a little different in XCOM as it sort of proved that there is an audience for factors of luck/unfairness in games still coupled with the turn-based gameplay that has been absent for a long time, although I do recommend spending quite some time between playthroughs of it though. I hope some of the quirks can be patched. I can't advice on Sleeping Dogs as I've not played it.
Those games are wayyyyy to different to really make a recommendation. Of the three I had more fun with Sleeping Dogs but I also am biased because open world/sandbox type games are probably the ones I like the most. If you want to play a good tactical game that is about planning tell her X-Com, if you want to play something "similar" to a GTA go with Sleeping Dogs, and if you want to play a Bioshock spinoff Dishonored is your choice.
I can personally attest that both Sleeping Dogs and X-Com are great games.
Also if she wants to save money I think X-Com might still be on sale with steam, as well as Sleeping Dogs. Dishonored not so much.
I haven't played XCOM yet, but between Sleeping Dogs and Dishonored, it's an easy choice: Dishonored. That said, I'd expect your assessment of XCOM is accurate based on what I've seen and heard, so if you want best bang for the buck, that'd be the way to go.
What didn't you like about Sleeping Dogs?
I haven't even played XCom but there's no way it can be worse than Sleeping Dogs.
I voted for Sleeping Dogs, but I haven't played the other two. Still, I don't think you could go wrong with Sleeping Dogs, I only beat it a month ago but I'm already wanting to do it again.
I haven't played X-COM yet, but I'll say X-COM anyway. Why? Because that's so me.
Sleeping Dogs and Dishonored are both great games, but if you only can get one and just one, the won't feel like they are enough. You can finish Dishonored in around 10 hours if you don't play very steathily, and Sleeping Dogs in around 20 if you stick to the main story quests (and possibly some of the more important side stuff). But X-COM? Sink your life away, duder.
I really, really have yet to love my time with XCOM. I'm generally terrible at strategy games, and XCOM isn't helping with that very much. It's very well-made and most people seem to adore it.
Sleeping Dogs is really neat, but I wouldn't say it's special. If you like games like GTA and Saints Row, this is definitely a good one of those games. But it doesn't hold up against the best games in that genre.
Then there's Dishonored, which is fascinating and I still think about a lot. That would be my pick, for sure, and to someone who doesn't play strategy games, it'd be my recommendation. If you do play strategy on the reg, though, XCOM is what I'd pretty much have to recommend to people.