If your hardware and software budgets are unlimited, AIX on IBM P6 hardware will perform better, but for a given amount of money you can buy more AMD/Intel processors and more AMD/Intel DB2 PVU licenses than you can on AIX and P6.

The PVU pricing is supposed to even-out the differences, but I still think for a given amount of money you would come out ahead with Linux on AMD/Intel.

I have run Linux on a P5 IBM box (not my decision), and I don't know what the performance difference is between that and running AIX, but probably not much. But If I had a P Series box, I would prefer to run AIX because that is what the vast majority of other customers are running on P-Series hardware and there are likely to be fewer bugs in that particular DB2 code.