Well, the road jerseys have that round bead of elastic at the bottom hem like a lot of road jerseys, and the mountain bike styles have a t shirt style soft bottom hem that lays flat and is maybe more comfortable? Otherwise, there's little difference except the individual styling diffs, pockets, etc.
I wore one of the summer weight full zip ones this summer for five days straight on a 450 mile tour with temps ranging from 55-98 + and was comfortable the entire time.
I bought them all out of their web outlet or their weekly sales.
Now, if you haven't tried wool cycling shorts yet, Ibex makes the best cycling short on the planet IMO.