Both AmigaOS4 and MorphOS claim to be "Amiga compatible" or more specifically have backwards compatibility for running Amiga 3.x software.
The question really is, which OS will be MORE compatible.
We have no idea how compatible AmigaOS 4 will be since the only thing shown t the public is Amiga OS 3.x with some 4.x elements. I expect the compatibility to be good since they will simply use the same 3.x libraries. However these libraries will still be 68k and will run under emulation. I would be shocked (but pleasantly so) if they released a fully native AmigaOS 4.
MorphOS wrote all their 3.x compatible libraries as PowerPC native from the very beginning so I would expect it to run old software much faster.
As for the interface both OSs will have interfaces which are different than AmigaOS 3.x. Until we sit down and compare 3.x, 4.x, and MOS I don't see how anybody can pass judgement.
The only thing I know is AOS4 isn't going to be running on new hardware any time soon. I would expect no earlier than 2004 at this rate. MOS, to Amiga Inc.'s shame, is far farther along despite the fact they had to write a lot of the OS from scratch instead of rewriting an old OS.