Looking at this site here:
http://www.partsforscooters.com/PFS-Chain-Chart
I was able to see a difference of thickness between #25 and #25H chain (roughly .24mm per plate, for overall thickness of about 1mm).
I am looking for a side-by-side tensile strength comparison to determine if it's truly worth making a jump from mcmaster #25 to a #25h change. If I find one, I'll post it here.