I agree that all of Pete's points about the differences in tire construction are valid (although the UV thing doesn't matter if you're going to replace them every 6 years as currently recommended anyway), but (other than the UV) they basically amount to a pressure rating.
A load range B tire of ST type may be inadequate for load capacity, and won't be much stiffer than a 35-psi P tire. A load range C tire of ST type will be stiffer (which is likely a good thing), but primarily because it is a 50-psi design... extra-load (XL) P-tires and both LT commercial tires
in load range C will be similarly constructed, for the same reason (and the same pressure).
My van has two sets of tires
of the same size: a P-type summer set with 35-psi rating, typical capacity for their size, and one body ply; and a winter set with 50-psi rating, higher load capacity, and two body plies.
Yes, stiffer tires
seem like a good idea, and that's the way I'm going with my next set for the Boler
. ST? Maybe, and it's the easiest type to find, but it's not the only option - even worse, an ST of the wrong (too low) load range won't have the desired construction. If you want stiffer, look for that higher load range (C or better).