The sewed on belt loop, IMO, is superior to the 1-piece sheath in that if anything ever happens to the belt loop, you can simply pull the stitching and apply a new one. With a 1-piece, if you cut or damage the belt loop...you're pretty much screwed.
That, and it looks more professional to me. I've made a couple of the 1 piece designs, per customer request, but I'd rather do the other.