Yeah, a 260X is pretty much just a slightly updated version of the HD7790. The 7850 would likely be the better-performing option. There are other things that might be worth considering too though, like whether one of the cards you are looking at has more on-board memory, a better selection of ports, or a better cooler.
I really don't think that the 7850 being a bit older would affect its performance to any notable degree, although it's possible that an older or more heavily-used card could potentially wear out sooner. On the other hand, the 260X being newer means it launched at a lower price point, so the manufacturer may have potentially cut more corners on the durability of some components. It's difficult to say either way though.