I have worked for companies that used both. Based on those experiences:

Use Power RISC with AIX in established IBM user organisation that wanted to run Unix software so we ported existing software to AIX.

Use existing s390 system running several core systems, to run Unix partitions to deploy software already written for Unix in C.

I have never even heard of any company not already being an IBM account migrating to either of those systems. The capital and operating costs are typically far higher than for comparable performance x86 based deployments. Technical staffing is much harder still.