I have a FW, two 99's, and also a 185 which is mechanically the same as a 99 but with a more modern body style.
Once properly cleaned and oiled, they are all fairly quiet-running, but they all sound different. My FW has a quiet, mechanical sort of "ticking" sound it makes as it sews that I actually like. The 99's have more of a "whirring" sound, but again, not unpleasant and not loud. My 185 is quietest of all I think, but it's also mounted in a heavier box than the 99's so that might be muffling some sound. If any of them make any loud (or loud-ish) noises I start looking for lint and dry gears.
99's are great little machines and I'd say when comparing them to FW's they fall short in two areas - one they're a LOT heavier, and two they don't have the little fold-out bed. On the other hand, I think they're stronger so if you're doing anything other than piecing quilting cotton, the 99 might be a better choice. They also have a bit more harp space than a FW.
99's are a LOT more available, you can find them very cheap if you look around. My two 99's and my 185 all added together cost me less than 1/2 what I paid for my FW, even after figuring in the cost of replacing the motor and light on one of the 99's.
Both are low shank so accessories that fit one should fit the other.
As a sewing machine junkie, I want to say just get both, you'll figure out which one you use the most in time.
