Consew doesn't make the servo motors, they just have their name put on it. From what I've seen and heard, they aren't very good.
I just looked at the site for the Nick O Sew, and I'm confused why they are talking about a "spare brake shoe" that goes with the motor. A true servo motor is a constant torque device with infinite speed control throughout its speed range, and doesn't need a "brake shoe" like a clutch motor.
I have been using Reliable Sew Quiet motors for about 6 years. I have two Sew-Quiet 3000s, and a Sew Quiet 4000. I have not had the tiniest problem with any of them, and one of them is running a very heavy duty machine. They now make the Sew-Quiet 5000 which is even better than the first two models. The 5000 has digital speed settings, not analog like the two you mentioned, and it is brushless.
I sew every day for a living, and need something that works. The Reliable servo motors are the only thing I will use. They are more expensive, but they are the best value as far as I'm concerned. Here's a link to the 5000: CLICK HERE