Hey, pass the David Boston Kool-Aid, bro!
Yes, Boston has more upside potential than Evans because of his freakish combination of size and speed. What are the chances of him reaching that potential at this point? Maybe 5%. The guy is constantly injured or doing something ludicrously stupid. True, Evans had knee problems in college, and that could easily come back at some point. But right now, I'd take Evans over Boston seven days a week and twice on Sunday.
I would note, however, that Evans vs. Boston isn't the relevant comparison. It's Booker vs. Evans. Evans and Booker are starters. If you want to compare Boston to his opposite number in Buffalo, it's either Josh Reed (who, as of 7:41 am on 5/19, has not been cut, but if you wait awhile, that might change) or Roscoe Parrish.