Remember that childhood game where you try to decide which famous person--or which book, or whatever--you'd like to have with you, if you were to be stranded on a desert island?
Well, choosing a processor is kind of like that. Except that, with a processor, it's not a game. Once you've chosen a processor, and designed your hardware and software around that processor, it becomes very expensive--and very time-consuming--to switch to another processor. So, you're likely to be stuck with whatever choice you've made for a good long time: five years, easily; maybe more.