Drew, both lathes are good lathes, I have a Jet 1220 VS in the shop. I do most of my turning on a Delta, but there are some projects that I do on the Jet. The obvious difference between the lathes you are looking at is that the 1221VS has reverse and the 12" swing would would be better if you were thinking about turning bowls, etc. The 1015 VS would be adequate for turning strikers and pot calls.
If there is a Rockler store nearby, you might be able to look at them to compare