A TT is capped at a weight of 10k, a 5er isn't (though you might need a DL endorsement over 10k or a different class over 15k depending on where you're living). A fifth wheel hitch will rip out of the bed of the truck before it comes unhitched in a collision. Having the hitch over the rear wheels of the truck v. the rear of the truck makes driving, maneuvering, & backing easier, IMHO.
You don't see many TT with more than one slide out.