What's the difference between regular and light 5th Wheel?

That whole "lightweight" thing is a bunch of malarkey, in my opinion. I think in just about every case, you'll find that the "lightweight" units are simply SMALLER.

As you point out, construction is virtually identical. Take a closer look-see at those numbers; I especially encourage you to add up the cubic footage, then divide that number into the weight. Weight per cu. foot is likely nearly the same for regular and "lightweight" models.