The Xtreme model has slightly higher Base/Boost clocks and also has an RGB logo on the backplate, while the regular card only has the top RGB lighting.
As for the OC-ability, they should be nearly identical. One thing to keep in mind is that the vast majority of Pascal GPUs all overclock to within a few percentage points of each other. The only serious difference in overclocking potential you get by buying one of these aftermarket cards with their various voltage measuring points and overbuilt power delivery systems is when you go to the extreme end of overclocking with sub-ambient cooling (LN2/LHe). If you aren't going to be doing any extreme overclocking, then you're really just left choosing a card based on the cooler type (blower or open air), warranty, and looks, and not for any additional OC potential.