Hub doubling, as in doubled dies or
Machine Doubling?
Machine Doubling - easy. The obverse die is the hammer die, thus comes loose easier.
Hub doubling - primarily because more people look at the obverse of their coins. There are almost certainly more nice doubled die reverse cents that just haven't yet been found. To answer the question, though...even chances - 50/50 obverse to reverse. The dies are made in the exact same manner.