Hi,
Can't speak for all makes but, on my Oakleys, the MX goggles have the little doobries to take tear-off visors while the snow goggles don't and, iirc, one version of the MX lens gives almost 100% light transmission, which isn't available on the snow goggles.
Fwiw, I used the Oakley MX goggles skiing last year because I specifically wanted a completely clear lens - I went to Finland and the previous year I'd used tinted goggles that were completely useless in low light - couldn't see the lumps 'n' bumps in the snow.

At least with clear-lens goggles, if it's sunny you can wear a pair of shades inside the goggles.
Hth.
Regards,
Stuart
I speak my mind, which is possibly why you don't hear much from me.