The 320 is a free arm, all metal but with an external belt driven motor. I've noticed this model for its' very nice wooden extention table and well as the free arm. I think the 431 has the direct drive motor, all metal machine in combination with the free arm, but I need to look it up to verify the motor drive. Many of the free arm models had internal motors but were belt driven. I'm not sure how available they are in the US. Except for the tiny Featherweight 222, Bernina tend to have the narrowest free arm and are very trouble free machines, but they are belt driven. If your requirements aren't absolute, I think some of the the green Husqvarna Automatic series are worth a look too.
Last edited by Mickey2; 01-18-2016 at 03:14 PM.