There was an interesting discussion about specs recently on the Rotorheads Forum. Worth a look.
I've nothing sensible to say except choose very carefully and with the intended purpose in mind (even if you have to buy "social" specs and "flying" specs).
I'm a new specs wearer and bought the pair that looked best - small ones. So I sit there in the cockpit with them clamped in a fixed position under the headset and when I glance down to the chart/approach plate on my lap, I'm looking under the lenses, not through them.