An additional difference is that Clayton is growing rapidly and Hillsborough (due to its location in slow growth Orange County) is growing slowly
Could this have anything to do with the philosophies btw the 2 different counties? Orange Co has always maintained a rather more conservative growth, mainly due to its concerns for its resources, namely water. Wake Co for example is now paying for (literally!) their exploding growth. Hillsborough is one of 3 towns in Orange County, and all have been fairly conscientious to maintain stability of resources. Hillsborough esp has learned from the mistakes of neighboring towns and counties, and is very cautious. Moreover, if you are unfamiliar with Chapel Hill there is a rural buffer which prohibits further growth. Chapel Hill is literally just about at its boundaries. Hillsborough has more room to grow, but right now has little infrastructure to annex more areas, and they recognize that limitation. Again, just being more cautious and not wanting to bite off more than they can chew.
As the pp put it, the choice btw Clayton and Hillsborough really depends on what your desires and expectations are. As for demographics, I'm not sure without checking the town websites, but I see lots of mid 40s with kids running around. Also see many older folks too. Also see young couples--just depends what section of town.