If they are both reasonably priced, in good condition, and work - hey boy, buy them both!
hmmmm... come to think of it, I don't have a 201 (my daughter has my mother's). I do have a 15-90 (the feed dogs drop which makes it great for free motion quilting).
The 15s have a bigger bobbin - the 201 is a 66 type and some consider it the best Singer ever made.
MOO - if possible get both.