Personally, I don't see much of a future in MOS: from a technical point view it targets the same restricted hardware as OS4, and feature wise, there's not much difference between the two.
On the marketing side, it lacks the Amiga name and that makes a big difference. As it is, it's just another OS that runs on the Pegasos platform.
OS4 is the new AmigaOS, and that fact alone is enough to raise a few eye browses. If Amiga and partners remain commited to it, deliver the promised regular updates and follow the path to OS5 as they scheduled some time ago, it's probably the most likely to suceed.
AROS is developing very well and at this pace, it seems likely that soon enough it will be in a usable state. The fact that it's free and runs on x86 can attract a lot of curious/nostalgic ex-Amigans to it. On the other hand it lacks the backing of a company to market it.
So, IMHO and if I have to predict the future:
Today - OS4 is the one to place your bets on
In 2-3 years - Both AROS and OS4 will be kicking ass