Semi Floater vs Full Floater
First off. I have seen 14B in 6 lug and 8 lug both SF and FF versions. So lugs nuts are not the factor here.
A FF is similar in design to a front axle. The axle will still roll and operate even if the axle is busted or removed. A FF supports the weight of the vehicle on a hub assembly and the axle is only used to transmit power from the differential to the wheel.
A SF uses and axle that has a bearing integrated into it as an assembly. SO, if you break an axle you aren't going anywhere. They are inherently weaker because the weight and stresses placed on the wheels are supported by only one set of bearings, and in my experience the bearing are smaller as well.
If you are going to spend money on a rear axle for your rig.. then go 14BFF and don't even consider the SF. BUT, if you are determined to go SF I bet there is a long list of people on this forum that have them sitting around after having swapped in a FF.