A featherweight has a regular belted motor. As does any 66, 99, etc that you will find. The 201-2 and the 15-91 have potted motors, the 201-3 and the 15-90 both have belted motors.
Simply stated, if the machine has an external motor and has a belt, it's belted.
If the machine has what looks like a "pod" or growth higher up on the arm (connected via the collar around the shaft that leads to the handwheel), and no visible belt, it's a potted motor.
The benefit of the potted motor is that it's "Direct drive". There's a gear that attaches to a gear at the handwheel and there's no belt to stretch or slip. This means that the power is more consistently given to the needle, and no belt to adjust.
The drawback of the potted motors - the gear that goes around the handwheel is a fiber gear. After 60 - 80 years, they can become brittle, especially if they're not properly greased. At last look, they were very difficult to find to replace a broken one.
In practice, I've found that my 15-90 would power through anything that my 15-91 did, so I don't think that the "piercing power" is that different, but it may have shown up more if I'd tried it more, for longer periods of time, but my need wasn't that great. It's why I often suggest a 15-90 when people really want a 15-91 for FMQ. You really don't need pure grunt power for quilting, and they're not that different in the end anyway. I quilted a 5'x4.5' quilt with a 15-90 and there was no complaint whatsoever from the machine.
The wicks, if they're crunchy or black, I tend to change them, but otherwise, if they're damp and the lube has penetrated all the end of the wick, they're not too much of an issue. The ones for the 15-91 and the 201-2 are easy to find. Some of the others aren't as easy. I posted here about wicks for some of the other motors:
http://www.quiltingboard.com/vintage...s-t205939.html