I can't speak for OS4, I don't have any access to the betas. But MorphOS is a mature, stable, polished Amigalike OS with only a few very minor niggles left (and I get the feeling that they're already fixed but right now only the core developers are allowed to have the updates. :-P). Its 68k<->PPC layering is very well designed and you can use Amiga 68k apps totally transparently without knowing or caring what they are. It's 68k emulator is incredibly fast and even faster with Trance (Hotspot JIT) running. You can even run Amiga WarpOS, Warp3D and PowerUp applications with no changes. Q-box sandboxing offers a future where MOS will be a fully capable modern OS with *true* memory protection and all the features expected these days.
MOS's biggest weakness is Ambient, its weak and undeveloped Workbench replacement. Apart from that, it's superior in every way to classic AmigaOS and the ultimate upgrade. MorphOS is an extremely well designed, well implemented AmigaOS clone with all the design features that anyone wanted from a PPC AmigaOS. OS4 will have to be *very* good to come close.
Oh and MOS will get PPC native TCP/IP stack, AmiTCP5 when 1.5 is released.
