Semi float (SF) has wheel bearings like a 12 bolt which mount in the axle housing and the axleshaft passes through it. Since the wheel hub is attached to the axleshaft essentially the weight of the truck rides on the axleshaft, a bad thing. Full floater (FF) has a wheel hub with 2 sets of bearings, similar to your front wheel hubs. The axleshafts pass through the hubs so no weight is resting directly on them which can also improve transfer of horsepower to the ground, hauling heavier loads, running bigger tires, etc. Another difference is ring gear size, 9.5" in the SF and 10.5" in the FF. Also the pinion in the FF has it's own removable housing, allowing easier pinion depth adjustment, eliminating the need to remove the carrier to access the pinion, and when installed the nose of the pinion has a pilot shaft which makes it stronger than the SF, kind of like the Ford 9".
I run 15" wheels on mine but it has to be converted to rear disk brakes (you lose your park brake too) and you have to get 15" 8 lug conversion wheels. I think the aftermarket makes 6 lug hubs for the FF but it is really cost-prohibitive. conversion wheels are $70 a piece, and one hub is likely 2-3 times that much.