I have a 1934 99K, with original motor and lights, it was rewired a few years ago and it runs fine. My machine is oiled regularly and motor was regreased at the time of the rewiring, and it's not a noisy machine. I think they are all much the same when clean and lubricated. They come in three versions, the earlier has a knob for stitch length regulating, the two later version have a stitch length lever with backtack. I don't think there's noticeably differences between them regarding noise. I have a 201 with a belted motor, and it's not very far off from it when it comes to noise, the 201 is on the quiet side. If the 99 is working very hard there are vibrations, but with light to medium, even heavier fabrics it's fine. Flat felled seams on regular jeans weight material is no problem. It's when you push it's limits the vibrations starts to appear, but it will stand up to quite a bit of mistreatment.
Over the Featherweight the advantages would of course be size, and it's a built more sturdily. It's much heavier though. It has drop in class 66 bobbins, easy to find and easy to thread. I know some find the bobbin case on the Featherweight a bit fuzzy and problems with nesting, once sorted out they seem to work fine though. I'm not sure how you like yours regarding issues with tangling or changing the bobbin. The 99 is more solidly built, it will handle anything you can fit under the presser foot. It uses the same feet and accessories as the Featherweight. The only thing about it has to be the adjustment of the a setting in the bobbin area; it's not easy to get it right with out the service manual or looking up ArchaicArcanes guide for it.
I don't have a Featherweight to compare with unfortunately, but with the 99 it's all about oiling and cleaning, and paying attention to the under side of the case. The feet have often crumbled away, I have fitted large pieces of furniture felt pads in each corner and one in center. It works well. Placing the machine on a solid table with stable feet is a must, but that goes for all machines. Singer used to sell a rubber mat for the 3/4 size machine, I'm sure something similar can be cut out but I'm happy with the furniture felt. My great aunt used a piece from a cotton blanket. If
this video is anything to go by, the Featherweight has nothing on my 99K when it comes to noise.