FWIW, the "greatest sewing machine ever made" competition, late '40s early 50s, in the UK, declared a tie between the 201 external motor, and a Necchi model. Singer, of course, painted with a broad brush, and in its advertising of the day, declared the 201-2 (potted) was the winner.
I have not yet finished refurbing my 201-2, but I have used a 15-91 potted, and a 15-x (not a -90) external motor, and much prefer the 15-external (RAF decals, 1936). The RAF walks through thick layers easily, and never balks. It was a well used machine, "broken in," as it were.
I have also found it much more likely to find a 15 with good wiring than a 201, but I live in an area where 201s are much less common. I believe 15-s were made much later than 201s, (tho other than the 15-75 and 15-125, I do not know end date of production on the 15-91.)
I have two 66- with back tack, and found they seem to have a vibration at higher speeds. The 99s I have are very sweet to sew with.