I have an Activa 240, the top of the line which preceded the 3 series. Love it! I can change the bobbin one-handed, under the table, and I don't have any special dexterity. Same solid, powerful, reliable, precise machine as my other Berninas. If it were me, I'd definitely go for the 350 over the 215.
Not sure what a "kick back" feature is. "Back stepping" is reverse, but so precise it goes in the same needle holes as the last several stitches.