Hi
I think Reaper pulled Linux support and honestly I did lots of reasearch into Linux for production work. There is a lot of promise but when you sit down to actually do something it all seems to evaporate. (BTW: I love Ubuntu as well)
Honestly the X series makes more sense from a UI perspective than 8.5 did. But it does make Sonar X a completely different DAW so just be ready to invest the time in the learning curve. It will pay huge dividends. Also be aware of how comping works in X3 it is very different than layers in 8.5 (layers were replaced with a feature called lanes). This seems to be a source of endless chatter on the forum. Some like it and some will never forgive Cakewalk over it.
Pro Channel is great, and addictive by the way, but like everyone else said pay attention to the version you are buying and make sure you get the features you want.
Good luck,
George