For an extra $80 bucks! That should be a no brainier and I would definitely buy the J fare, just for the benefits of J alone.
I'm pretty sure FM/MU are pretty much on par in service, FM might be a little friendlier but they are pretty much the same. MU might have the newer J seats on their A320's though. I'm sure others on here have more insight, but if u can I would pick the A330 flight with a proper J seat.