36r = heavy duty 5 leaf spring
14f = medium duty 4 leaf spring
The 36r spring will handle more weight (hard top, winch, lots of armor, heavy tire carrier, etc). 14f spring will work well with lighter rigs (soft top, light armor, etc)
I run the 14f springs all around with a winch, tire carrier, heavy rocker guards, soft top and doors and it hasn't sagged at all. Great ride on and off road. Very flexy as well.