I don't think Xenonauts will be outright in competition with Firaxis' game. Having read the forums over yonder, and in various places on the net, there is an awful lot of whining about how Firaxis are "ruining everything". Given that Xenonauts seems to have stuck a lot closer to the vein of the original, I reckon that those dissatisfied with X-Com:UE, or those who just don't like the look of it will turn to Xenonauts. Likewise, people who are impatient for a taste of X-Com style action might end up pre-ordering this to get to the Alpha build (which is a pretty sizeable advantage over the Firaxis project).
Then again there are also people, like myself, who will buy (and probably enjoy) both games. Rushing the game to release before its mega-budget "rival" will hurt more than it helps most likely.