Hi
Both are a great solution for pSeries box.
If you are looking for an user friendly and a easy of admin solution, SuSe and its YAST is the answer
red Hat includes LVM at instalation time that its good for people who loves
AIX.
But in the daily job i have no preference.
Only one recomendatión, if you want to install Oracle DBS, IAS or any Oracle Product in a pSeries partition (or micropatition) use
AIX, its a realy headche to installOracle in Linux, finally it works fine, but it is very dangerous for your health.
Good look