Which is better Rega Planar P9 vs RP8

The RP8 is a replacement to the P7 not the P9; I believe that Rega will be bringing out an RP10 to replace the P9.

Only time will tell. In many cases newer is better, but in many cases it's not. I'm sure the manufacturer will try to convince you that the RP8 is better than the R9, that's their job.
I've also seen cases where the top of the line was not a manufacturer's best product, they do usually figure it out, after awhile.

I would hope the RP8 is better since it is far newer in design and replaced the P9.