I would choose the 15-91 only because I already have a 201-2. I'm one of those few that don't care for the 401s.
The cabinets may or may not be interchangeable. The 15 and the 201 will fit in the 401 cabinet with a bit of empty space to the right of the machine. Starting with the 401, Singer machines were a little bit longer than previous machines. Hinge spacing is the same though. If the 15 and/or the 201 don't have that little bit of empty space to the right of the machine then the 401 won't fit.
Cari