Functionally they are about the same. Ergonomically, I differentiate.
I have a preference for the XS/Thermo valves for bigger heavier tanks; Blue Steel/San-O-Sub on pony (40 cu ft and smaller) bottles.
The BS valves have a shorter handwheel stem and a big honkin flared base. On heavier tanks I find the BS squeezes my fingers more when I carry them and it kinda hurts. Thermo not so much as the wheel stem sticks out maybe a quarter inch further and the base flange sits flatter on the top of the tank neck. More room for fingers. On the pony bottles the shorter wheel stem is a positive since it does not stick out as far from the tank wall as the Thermos do. Also, as the pony bottles are lighter, it doesn't pinch as much.
On big heavy tanks like the 100s, if they came with BS valves free, I'd pull them off (then say "but I had two valves - I HAD to buy a pair of 19s!") and buy the XS/Thermo valves.
OMMOHY