A good pair of goggles would undoubtedly offer the very best protection against the sun (as it covers a large area of your face), it is just getting way too hot on the slopes sometime to wear them, especially during the spring ski season. Sunglasses are the next best option of course, but you have to make sure that the shades you choose qualify for use on the snow especially when the sun is moving higher and higher in the sky. Never go out without eye protection as it is not uncommon for people to burn their corneas when they don't wear sunglasses (or ineffective sunglasses for this matter) on a full day out on the slopes. The snow is reflecting light back so you are getting hit from above and below.
In order to choose good sunglasses for use in the snow there are several aspects to take into consideration. First of all, of course, you need the best lenses you can get. For spring conditions, I highly recommend going as dark as possible, and I personally would always choose polarized lenses that guarantee at least 95 percent protection against both UVA and UVB rays. The best choice, in my opinion are sunglasses that have interchangeable lenses, so you can quickly swap them out in case things get foggy or the cloud cover calls for lighter tinted lenses. If possible try and get impact-resistant lenses so they won't shatter if you accidentally run into a branch during tree runs or when making one of those graceful face plants.
For skiing purposes, choose shades that cover a large portion of your eye area. Ideal are glasses that feature side-shields to keep the sun from coming in peripherally. Sunglasses with wider cut temples are more effective in that way also. Wrap-around lenses work better than flat ones as they block more of the light coming in from the side.
Then, of course, there is the looks and coolness factor. I happened to like all the sunglasses reviewed below. Looks-wise, some are more geared towards functionality while others are designed to impress, but all the shades listed below are great choices and will protect your eyes. My recommendation is to go to the store and try them on. If you wear a helmet skiing, bring it along to make sure the glasses fit with it.