what sled is better 1999 670 mxz or 1999 670 summit ?

Depends what the your riding conditions are like...

Thats like comparing a truck to a sports car. They are made for a specific type of riding.

If you trail ride then go for the MXZ if you want to climb then go for the Summit.

I would say the MXZ I think in 1999 you could only get the 670 with H.O. am I right...if so..I really like them with those nice 44mm carbs just a pumpin :D ...but like as said if you like to climb buy the summit if you like to trail ride and a little ditch banging buy the MXZ

The only Summit available for 99 was the X with the "HO" motors, 44 carb and all.