If you can afford it, you should definitely look at AmigaOS 4.0 as it's by far the most complete AmigaOS I've used (in as much as it comes with compilers, SDK etc.) and is a very nice OS to use and code for.
The only problem is the opposite of AROS's main benefit - availability of hardware. AmigaOS 4 only runs on AmigaOne machines which are like hens' teeth. When somebody brings out a new machine, though (Sam?) be sure to check it out.
It does of course have the large advantage of being almost completely compatible with AmigaOS at a very good speed (AROS requires recompilations - you can't run Amiga binaries on it).
Unfortunately there's no easy way to test AmigaOS 4 though as hardware is impossible to come by and there's no emulators, but when hardware arrives I think it's the best solution if you can afford it. The Hyperion guys have done a fantastic job on it.
