Your design code will most probably tell you this. Since you mentioned SA240, I assume your work is under one of the ASME BPV Sections, correct?
If true, have you looked up the applicable material notes (G, H, S, T and W) in section II-D?
Perhaps both 310 and 310H are possible, but one with restrictions, making it more appropriate to use the H-grade (thats what weve found over the years with similar materials).
If not, please tell us your applicable design code.