Which Is the Best Way to Weld P11 with P91 Pipes?

Dear Sirs, I read several methods to join a P11 [1.25 Cr- 1/2 Mo] pipe to P91 [9Cr - 1 Mo] pipe, such as : 1. Weld using filler metal corresponding to corresponding to [2-1/4Cr - 1Mo] steel & post weld heat treat at appropriate temp.. 2. Butter P91 with filler corresponding to [2-1/4Cr - 1Mo], post weld heat treat & weld with P11 & re-post weld heat treat it. 3. Use a P22 pup piece to join P11 with P91, etc. etc..... However, I wish to know, which is the most appropriate method taking into considerations the overall weld-integrity and high temperature strength of the weldment. Please help to clarify my doubts. THANKS, DDN VERMA