As I see it...
1. Emulated applications- AmigaOS4/MorphOS emulate 68k applications by calling native PowerPC libraries. As there's no integrated emulation for custome chips (except some basic stuff), most hardware-banging software won't run (and may easily bring the OS down).
68k applications that will work this way will be real fast (a lot faster than when run through UAE).
- AROS will call UAE when encountering 68k applications. The first thing it implies is that UAE requires a full AmigaOS 68k install or at least a compatible kickstart for simple games/demos while AmigaOS4/MorphOS don't have such requirement. So today, (freely) distributing AROS with the option to run Amiga68k applications through the UAE integration isn't possible. A kickstart replacement has been announced, but it's been 10 years at least since it's been announced, so I wouldn't hold my breath. So this is an obstacle for me.
The second difference is that UAE will run 100% of the 68k applications, not only "OS-friendly" ones, since it will emulate every custom chips. And a crash of a 68k application will only crash UAE, since the emulated application doesn't have full access to the OS'resources, unlike AmigaOS4/MorphOS'transparent emulation.
That's the big differences I can see.
2. Native applications VS emulated onesSpeedwise, some emulated applications may run slower on AROS since the emulation is a lot heavier. But native applications will run a lot faster because the x86 will be a lot faster than PPC (at least today's x86 are faster than the fastest PPC OS4/MorphOS run on: who knows ? This may change in the future
).
3. CompatibilityThe 3 OS are binary incompatible: an AmigaOS4 executable won't run on AROS, nor MorphOS (well, could be run through OS4Emu but the author decided to stop its development, so let's forget about this option), and an AROS executable won't run on MorphOS nor AmigaOS4.
All 3 OS are source-compatible, if you don't use any of the OS'specific functions/libs and of course don't use inline ASM stuff (like Altivec code or SSE code).
Last but not least, the 3 OS lack modern features such as Memory Protection, Resource Tracking, etc... And these features can't be added without losing not only 68k compatibilty, but also today's native applications.
Oh, and btw MorphOS and AROS aren't based on AmigaOS sources. They are reimplementation of the Amiga's libraries, much like Linux is a reimplementation of original Unix, but isn't based on Unix sources.
