You're always guessing if you pick someone on 1 season, especially if there've been several average ones beforehand. Obviously, if you have the choice between that and someone who's never done anything of note, you pick the former, but if you have the choice between Ambrose and Pothas or even Foster, you're mad if you pick the short-term qualified candidate over the longer-term one.
Yes, you get lucky every now and then, but that doesn't mean you should try it.
IMO Pothas, if available and committed to the England cause, should be picked for the First Test this summer, and if not, Foster should.