The 201 has been said to be the best Singer ever made. I don't think anyone has said the same about the 101. I don't own a 201(yet) and I gave my 1926 101 to my daughter. My daughter's 101 seems a little slow and possibly underpowered but that could be an issue with the motor too. It does run smooth and quiet though. Both are true rotary hook machines and use class 66 bobbins. The 101 is interesting as Singer's first electric only design. I don't know how many years they were made but I think it was a comparatively short time, most likely due to high manufacturing costs compared to other machines. I wouldn't say they are rare but they are not as common as other models either. If you want a 101 for your collection go for it though chances are you already own better machines. Not every machine needs to be 100% practical.
Rodney