I have a 99 and a 201, and they are both nice machines. I like the full size of my 201, and it's in a small table which means it very pratcial for maneuvering larger pieces of fabric. The 201 has revers stitch lever, and can lower the feed dogs. All 201s have the same features, even if the later ones look a bit different they work the same way. It's a very smooth running machine. Internally it has tree sets of meshing gears, and a rotary hook. It will stay smooth even if you floor the pedal, the only thing that gets louder is the needle punching through the fabric. The aluminium version 201K23 is noticeably lighter. I have both version, in a cabinet you will not think much of it, but in a portable bent wood case or a suit case type model you will notice the weight difference.
My 99 is the older version with the round stitch length knob. It's screws in and out and works perfectly. The bobbin case comes easily out on this machine, for cleaning purposes it's ideal. It is a bit trickier to get the bobbin out on the 201, but when you understand how it clicks into place it's gradually gets easier. The 99 is as mentioned a 3/4 sized machine, for most sewing I don't mind, but for working on jeans and jackets the full size models have an advantage. Model 99 became very popular and long lived (through the 185 too) because of it's size; so for many this is a pro.
The later 99s have a stitch lenght lever with reverse, and there's three vesions of the 99 as far as I know. They are all nice, but only the two later ones have the backtak lever. It's much the same with model 15; it had such a long life it went through all kinds of minor development. The later ones have the same stitch length lever as the 201, which is the only one with true revers; meaning it easily adjust for same stitch length both ways. The backtak on the 99 has a set stitch lenght. I have to admit, I like my 99 with out the revers, the only thing is I have to turn the fabric around to sew the other way. For daring and lots of back and forth I choose the 201.
The 201 originally came with a darning / embroidery foot. It's often missing in the accessory box, but this model works well with several of the modern low shank jumping feet. The 99 didn't come with any foot for this purpose, but a spring to hold the thread in place. You have to choose darning foot carefully for the 99, because the presser foot bar is a bit shorter and act up with some of the new ones. The 99, 66 and the older version 15 uses a feed dog cover, because the feed dogs cannot be lowered. The 15s with the stitch length lever can have the feed dogs lowered.
The all work equally well with the various attachments like buttonholer, and zigzaggers (well for me it's arguably how well zigzag attachments works, but the buttonholers are nice and I like my Swiss Zigzagger). Some of these features are more differences than pros and cons really, some are handy though like the stitch length lever.