AOS4 currently runs on the Amigaones XE,SE,u
It also runs on the PPC powered Classic Amigas.
Thought the classic version is not released to the general public yet.
It is being developed by Hyperion and is for sale currently only bundled with Amigaone hardware.
MorphOS works on the Pegasos boards. It is currently bundled on the hard drive with Linux, not with the CD or the Super bundle anymore. Portions of it are now being developed open source and available for download for free. It too runs on the PPC powered Classic Amigas. Thought the Classic version is not yet released to the general public yet. Guy's please, please do so soon.
Personal choice on the OS's really.
Reaction GUI versus MUI GUI
If you liked OS3.5 or 3.9 then you will love 4.0
If you liked Magic WB than you will really love Ambient.
kgrach
Also stay away from purchasing Amigaone SE or Pegasos I boards unless the price is right as they tend to be buggy hardware wise.