A nickel base alloy is preferrable (Alloy 625 is common) so that the thermal expansion matches the Grade 91 side more closely placing most of the thermal strain on the more ductile austenitic joint between the nickel and the SS (same reason you switch to nickels from 309 in higher temp applications for more common dissimilars between austenitics and ferritics/bainitics) .
There are also some special EPRI chemistries available specifically designed for Grade 91 to austenitics such as 347.
Give the guys at Euroweld a call. They pretty much know more about this stuff than anybody between the oceans.