Look around your shop. IF there's a dominant variant of Linux on Intel, try to go with that one. Will save lots of politicking if you can say "it's just like your Intel boxes". There may be some good overlaps on maintenance contracts too.
Application certification is usually the other deciding factor. Otherwise, the choices are pretty similar. RH does not market an HA product for RHEL on z, but an equivalent solution is available from SNA.
