Both, most definitely, don't wear expensive Sun Glasses as chances are they will get broken, but spend as much as you can afford on googles, cheap ones are pants, take it from someone who knows
Boarding or Skiing? I find when I'm boarding I always wear goggles, but when skiing I sometimes switch to sunglasses. Not entirely sure why. Maybe because I'm more likely to stack it boarding and as the goggles are attached I dont lose them!
I'd say it depends on conditions and preference. I used to wear sunglasses and then I went to Borovets and had to switch to goggles. I've not been able to use the sunglasses since. Borovets in Jan and Feb can be very cold and windy on Mussala and if you wear contact lenses like I do then goggles are a must i.m.o.