In the good old days PHE's lived by the 300/300 rule meaning that you could have 300 degF or 300 PSIG but not both at the same time.
I notice that today manufacturers seem to have raised the upper limit on pressure. Temperature is largely driven by the gasket material.
To answer your question, either plate thickness is common, but for higher pressure I would prefer the thicker plate. Some of my decision would depend on the pressure on the other side of the plate and the consistency of this pressure. If the pressure is fairly equal on each side of the plate and would mostly remain so, I would go with the thinner plate.
The plate thickness also has to do with the heat transfer characteristics and one silly millimeter might well throw the Hx into a larger frame.
There is a lot to be considered and not an awful lot of info to go in to offer you any better tips.
rmw