TransPac and TransPlate share the same design for the harness. The difference is the softpack vs a metal plate.
The metal plate adds rigidity and extra weight. The rigidity is definitely needed for large doubles, though many swear by it for singles too. The weight is nice since, if you go with the SS plate, that's approximately 6# that you don't have to carry on your waist. I agree with others that a plate isn't uncomfortable in the water. Wore a plate with .5mm, and felt great.
The softpack is padded so is more comfortable top side. Nice for long hikes with minimal exposure protection. For single recreational diving, I find the TransPac very stable. Actually packing for a vacation now, and am taking the TransPac and leaving my backplate at home.