Nice post!
I agree that Ubuntu’s USP for this really is the “convergent device”, which would really give the OS an edge over the others.
In terms on why you choose a mostly-Linux phone over Android, I really thought that it came to the fact their is no Java interpreter (Dalvik or ART) to need to keep running. To my understanding, this is one of the big reasons that you need to throw laptop-spec hardware into a phone (ultimately sacrificing battery life in the process). The fact by Bq with Ubuntu gets the same life as my G3 possibly shows how well Google have refined ART recently, but my Nokia N900 (running Maemo) lasted much longer.
Apps wise, I think web is the only way to go now. I would hope that building an app for each platform has started to slowly die. Android supports WebGL (and it runs very well), so even games can be done.