They are, to all practical intents, the same card, the R9 270X may be a tiny bit faster but you'll only spot the difference by running benchmarks, you'll see no difference in a game.
One key difference though may be cooling, both those cards do need a decent cooler to run at decent temperatures without excessive noise, so can you link to the cards you have in mind?
EDIT: Oops! Looks like I added the 'X' on my own, Prit87 is correct, the R9 270 is, indeed a HD7850 while the R9 270'X' is an HD7870, and the HD7870 is a fair bit quicker,
http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899-7.html just be careful to check it has a decent cooler if you choose that card.