I have both the 401a and the 500a. I like them both. My wife stole the 401 from me so I mostly use my 500 but to be honest I can't tell a whole lot of difference in normal straight or ZZ stitching between them. I do not do FMQ so I'll leave that to Candace and others who do.
With my three machines, one 401 and two 500s I've done skads of patterns with both the built in cam stack and cams. We have almost all the cams ever made for these machines and I use them quite a bit.
Both the 401 and 500 are noiser than some other machines, but that is due to their all metal ( steel ) mechanical insides.
Nothing a drop or two of oil once in a while won't take care of.
Internally the 401 and 500s are pretty much identical. The cams are the same, the available patterns are the same, the selectors are the same, only the outer shell, hand wheel, and thread paths differ.
The only thing I can say is decide which one you like the best and then ....... um buy both.
That's what we did.
Joe