My rule is I need/want every model of vintage SM, especially Singer, but have yet to add a 221/222 to my collection of 2x 128, 3x 15-91, 99, 66, 2x 403, 201, and a Spartan. The right priced 221 would be hard to pass up, though. I'm sure the 221 is a great machine but I'm not convinced it has an advantage, other than size, which you indicate is not important, over any of the above.
Ron