Originally Posted by silentlysailing

26 in mountain bike wheels are normally stronger than the 700c and have higher spoke counts. Also tire inflation plays a key and you need to find the comfortable balance with some cush yet no pinch flats and good rolling resistance. Personally from my experience tires make the biggest difference. A carbon bike is just asking to get stolen and I personally think the frames can be brittle.

If you are really worried about the unavoidable potholes I think a 1.5 to 1.75 would do good. Also does the mountain bike have an aggressive geometry? If so it could be about the same speed as a flat bar hybrid.