We avoid that combination if at all possible when the material is P91 (was P-No. 5B Gr. 2, now P-No. 15E).  Our applications of P-No. 5B are usually elevated temperature applications where P-No. 1 would not hold up.  But, we have welded 5%Cr (P-No. 5B Gr. 1) valve bodies to P-No. 1 using E9018-B3 occasionally.  This is for lower temperature design, below 750 F.

Marty is right. B3 is a good choice. You 'll find the MSTS of P5B's varies from 60ksi to 90ksi (some 9Cr castings and forgings are 90 while 5 Cr pipe is predominantly 60)depending upon product type and B3 would cover them all, though it doesn' have to. It only has to beat the CS.
In the old days you would see CS to Gr91 direct (talk about a PWHT razors edge) until we wised up (I always thought it stupid, but was a Gr 91 rookie back then). If its done today its either buttered or transitioned, and it ain't done often. In fact I cannot remember the last time I saw it used. But as Marty also stated 91 is no longer 5B so you are essentially talkin 5 and 9 Cr.
B2 can also be good and can reduce the strength gradient from the weld metal to the CS side across the joint.