From my experience, i think that steel is the way to go on a heavy service loading ramp. Why???
#1. Aluminum is only as strong as steel pound for pound. Thus, a lighter aluminum ramp just cannot be as strong as a heavier steel ramp.
#2. Aluminum is tricky to weld correctly (but, hey... maybe you have the proper mig or tig stuff to whip it out.. who knows?)
#3. Steel is much more easily repaired on the road. Remember, nothing breaks when you are home near you tools. So make sure whatever you have is dead simple to fix in a strange town, late at night. Everybody knows somebody who can weld steel... aluminum? not so common.
Here is a photo of the ramps we put under our 7,000lb trailers. the 10,000 and 14,000lb rated trailers use basically the same dimensions with stronger 3" channel lengthwise frames instead of 2" channel. they are 18" wide and 5' long.
MAKE sure you weld them up like they are going to be taking artillery hits. It's your butt on the line when you are up on those babies on a tractor!
Good Luck.
Craig the TrailerGuy