If you're going to the expense of buying IBM pSeries servers, then generally, in broad terms, you may as well run AIX on them which is specifically crafted to drive the hardware as efficiently as possible. That goes from the hypervisor down through to the adapters.
If you want to run Linux, you may as well buy xSeries hardware (in IBM terms, or Intel / AMD kit).
What you get with pSeries hardware in combination with AIX is powerful enterprise class processing. Yes, pSeries hardware + Linux probably gives you close to that, but you save yourself virtually nothing in overall costs.
It's hard to get specific details, but the numbers I've seen quoted are that AIX will run the same workloads on pSeries hardware 5-10% faster than Linux.
Don't forget, since Linux is essentially free, and your pSeries hardware can be easily partitioned, you could trivially install one AIX partition, one Linux partition and run your own benchmarks with your own specific workloads. IBM Business Partners have access to environments where they can run those benchmarks for you - and I strongly recommend you take your vendor up on that ability.
All this assumes you are truly intending to buy pSeries and run either AIX or Linux. If in reality you're thinking of pSeries vs. Intel/AMD based hardware then it's an entirely different ballgame, and the OS won't be the deciding factor in performance in that case.
Actually, I'd recommend AIX on pSeries any day due to the mature logical volume management (best in the UNIX world in my view) and the exceptionaly versatile virtualisation (again, in my view, best of breed in the enterprise UNIX arena).