You've framed (no pun intended ) the issue correctly… sure tortoise is a bit more elegant, black is sleek and minimal and well… black. It goes with anything. If you don't have a varied collection of shades I'd say go black… if you want something more unique then tortoise. I have about 10 pairs of shades from Peoples, Ford, Persol, Sama etc. and eventually you'll have one of everything.