Efficiency based on rated amps of belt drive motors does not apply to the 201K2, being a gear drive. The gear drive is so much more efficient since it does not suffer from the drawbacks of belt drives. If you can do it, measure the output at the main drive shaft, not the motor shaft. Measuring the motor output does not take into account the belt tension,and power losses result. At the needle my 15-91 has far more "power" than the belted model.

Leon- I don't have a 201 for a direct comparison, but it appears that the gear case is interchangeable between the 15-91 and 201k2..

Gymnast- You're probably right on the gear count of 10-48. I had no quick way to attach a degree wheel to the motor output, but spinning the handwheel 1 revolution produced between 4.5 to 5 turns at the motor. Hard to visualize the exact locations on the end of motor shaft, but close enough.

Measurement on the main shaft could give more reliable information. But we do not have that.

V-belts should have an effieciency of about 90-96 % according to this reference. But with belts the effieciency do depend on maintenance factors, so of cause it can be bad.
Book on transmission

Worm gears with a gear ratio of 1:5 should have an effiecency of about 90 % according to this reference:
Article on worm gear

So from a theoretical point of view, I do not find a big difference in the two ways of the drive. I guess, that the V-belt may have some more static friction. But in both cases the static friction of the commutator in the motor will be significant.

way off topic. Chevrolet Corvairs had fan/alternator belts that had two 90 degree bends in them .I always questioned their efficiency. Never owned one.

When I see the potted motor sold from ebay, the seller claims that the motor with drive can be used on both the 201 and 15-91. However you need to be aware, that the bobbin winders are different, because different bobbins are used.

Good to know, Thanks Gymnast2