One important factor to remember about sunglasses for pilots is that most of
us wear headsets these days. Any but the thinnest (and, preferably,
flattest) of frame temples can be a pain after a few hours of being pressed
into your head.

The most comfortable pair of sunglasses I ever owned had carbon fiber
frames. The temples were extremely thin and flexible, yet ultra-strong.
Match that with quality ground glass lenses (bifocals, of course, for us old
farts) in dark neutral gray (I think lighter gray in he near vision area
would be good), and you probably have as close to ideal as possible. Oh,
and by the way, avoid polarized lenses. They are good for cutting glare,
but since many avionics face panels are also polarized (for the same reason)
using them can turn these face panels opaque.

Why glass, given the weight penalty? MUCH better scratch resistance, and
can safely be cleaned in flight without fussing about what sort of cloth you
are using. Of course, if you require a severe correction (i.e. you are very
nearsighted or farsighted), the lenses have to be thicker and the weight
penalty for glass becomes larger.

Bottom line: if you have a big budget for prescription sunglasses, shop
first for the frames that fit best and feel most comfortable. Take your
headset to the optician's for a trial "under pressure". You may have to
visit several stores to find the perfect frames.

-Elliott Drucker