5" vs 8" I believe... this from memory.
I got the Mntn Skis. WELL WORTH THE $200 Cdn, for both.
Major difference in floation on my GT.
Be sure to get the extra parts you need. The bolts that go through the ski, the Mtn Ski rubber insert, washers etc etc. Dealer can help with this. Total was $230 for all.
Then I topped it off with a factory 6" carbide... it wasn't the stock one, factory upgrade I think, can't remember now, but best of both worlds.
Took some very mile modification to bolt on the skis... cough cough (hammer) but fits perfect now and works excellent.
-Steve