Comparing AROS and MOS is a little unfair. Both are good in their own rights.
MOS has the luxury of being a for pay product. While I doubt that is making anyone rich, even a little income can be a great motivator. MOS also has the luxury of having a certain select number of machines it'll run on. While it'll run on a lot of PPC Mac's, I'm sure developing it for known, consistent hardware bases is a hell of a lot easier than the piecemeal "whatever I put together" approach that AROS has to contend with. I haven't, and likely won't ever register MOS due to problems with select members of the community. A community I would assuredly need to deal with at some time. It is a hobby OS - the support structure involves me posting on forums and such, and I'm not going to pay for that pleasure when even just 1-2 of them are just abhorrent to deal with. Hate to paint a whole fence with one brush like that, but it is true for me. It only takes one turd in the swimming pool for me to not want to stick my toe in the shallow end.
AROS is a tremendous product in its own right, and undoubtedly gets the short end of the stick a lot of the time, even from guys like me. I toy with AROS quite a bit lately, but admittedly I don't use it all that much because of the select hardware it'll run on, and I certainly can't expect them to code drivers for graphics cards that are released every 10 weeks.
I see both as a labor of love, really - but there is absolutely zero doubt MOS is a more polished product from the users end. I've had great luck with MOS, and it supports legacy programs extremely well. If AROS could run on all my machines, I'd use it a lot more. The machine I have AROS on also runs as a Amithlon box, which between AROS and Amithlon I actually get more use out of the Amithlon side of that machine.