I am first and foremost, an RO fan. To me, without a doubt, ROOST is the best computer game I have ever played. It really has become THE standard for which I judge all other games, at least first person shooters. To say I am looking forward to ROHOS is almost an understatement.
KF is fun, more so when I get a chance to game with my son. But it is more mindless fun and while I do like it, it just does not in anyway compare to RO. In fact, I most likely would not have bought it except for the fact that Tripwire made it... I'm generally not into these types of shooters.
Understandably, ROHOS has been in development for years and is (sorta') a sequel to their first game released. With Steam more widely used today, I can see KF actually outselling ROOST (until ROOST came out, I didn't even know what F Steam was), but on this logic alone, I can see ROHOS doing even better, way better, especially since it will seem so different than most of the other games I see being developed and released out there.
So, to me, I don't care if KF outsold ROOST or not, just glad that the company is successful and because of that they will be able to continue releasing (hopefully) the type of games that I enjoy playing, that is a bonus to me and probably others as well. And if a KF2 is made, I will probably purchase it just because it is Tripwire.