Heya,
Completely different headphones. The FA-003 is harder to get, for one thing, so if you're not patient (like 6~7 weeks patient), then it's off your list. That said, it's easily the better headphone in my opinion, as it's a lot more natural, smooth, more neutral, way nicer sound stage, and fathoms more comfortable. Comes with two ear pad types (velour & leather) and have nice deep cups. The M50 is more bassy, less mids, and slightly harsher treble, very cramped sound stage, clamps hard on your head, and have pretty shallow cups and are only leather pads. If you want a headphone that is a beater and does bass and can be thrown down some stairs, get the M50. If you want something that will reproduce accurate audio, be comfortable, and be more along the lines of what an audiophile would seek, the FA-003 is the one out of your two (but again, beware it takes a while to get one shipped).
Very best,