I find it a bit sad (and funny tbh) that today we still have this schism, really.
How is OS4? It's like OS3.x only a lot faster and with improved features.
How is MorphOS? It's like OS3.x only a lot faster and with improved features.
How is AROS? It's like OS3.x only a lot faster and with improved features.
Which one you use is entirely down to personal preferences at the end of the day. If you prefer PPC, stick to OS4/MorphOS. If you want to go x86, go for AROS.
AROS is free and open, OS4 and MorphOS are not. If you are all about open-source, there is only one choice. Otherwise, there is thus the cost of ownership to consider. MorphOS has targeted the second hand Mac market, so hardware can be obtained cheaply. In the end, OS4 will cost you the most (unless you get a 2nd hand system), hardware to run it is still on sale as new.