Wow, awesome advice guys!
@JoeR: I think I may be confusing the meaning of "flat-light" - I thought flat-light is when light appears to come from every possible direction.
Sort of, but the point is that it's dim, not bright. You probably wouldn't feel "blinded" in flat-light conditions, at least not at first, even with no goggles at all. Bright sunshine and flat light are almost opposites, so one lens is not going to be really good for both.