Semantics, boudreaux. It's never truly a matter of need, unless a particular climb is crucial to your immediate escape from a roving death squad. I consider myself a climber, and I use a double with swagger, but in the end, I live at the base of a tall hill range and there are monstrous roads I don't (can't) do on my best day. Whatever. I just don't do them. I can do other climbs. Preference.
Pete, rest assured that either way you chose, you'll get by. Some riders (myself included) appreciate the relative simplicity of a double drivetrain -- but as a seasoned MTB rider, you probably couldn't care less about shifting through the extra trim positions, so that might be moot. There's an undeniable advantage to having an extra ring to work with, but you shouldn't worry that riding where you want to go will hinge on whether you've got a triple.
Pick one (maybe after doing some test rides, and getting a feel for the shifting), and enjoy it. Nothing wrong with selecting that triple. If you do end up with a double, though, and ultimately find that some hills are too friggin' steep, well then, you've met your match. Train harder, and if that doesn't work, either buy a triple, or find easier climbs. No shame in either.
No worries either. Have fun, take care.