The weight difference is a 99 weighs about 25 lbs without the case. A Featherweight comes in at around 11 lbs. Singer did make an aluminum 99 but they're rare. There are also some Japanese 99 clones out there that are aluminum.
I don't have a Featherweight but I love my 99s. If you fit them with a hand crank they're a favorite with kids.
I haven't noticed any extreme noise with any of my 99s.
The one advantage the Featherweight has other than weight is it is probably better for FMQ due to the vertical bobbin. That advantage might be negated by the very small harp space though.
Rodney