Most of the tinted lenses do the same thing sunglasses do. If you don't like the sun in your eyes, consider a tinted lens. Some are tinted different colors - like amber or orange or yellow... and some people like these for outdoor play because they act like shooting glasses and help you pick out movement in the shadows.
The mirrored lenses act just like tinted lenses, but some people prefer the look of the mirror.
For indoor play, you're best bet is a clear lens. Often times indoor fields aren't as well-lit as you might like and it's just harder to see what's going on around you. (Try wearing sunglasses in a dimly lit room to see what I mean.)
(Just in case you were wondering, the Proteus uses the JT Spectra lens... lots of different options for you.)
Emag / Bushmaster PDS / Trilogy Pro / 98c Flatline / Z1 / Z1 / Rental Z / Auto Hog / VM-68
Minimag (w/ pump kit) / Unibody Phantom / Phantom VSC / PGP / Sterling STP / King Cobra