i personally went out and checked these two bindings. ive never rode on them but when i saw the two i liked the 390 better. its really comfy, the toe strap really conforms to ur boot well, really really customizable, and the ratchets feel really solid. the unions are nice but when i compared the ratchets on the 390 to the unions, hands down the 390s took my vote. the 390s have a lot going on in the heel strap. the 390s didnt have any pressure points at all, where as the unions did. but it probably felt that way because i didnt have time to set up the unions to fit my boot well. but i am sure once dialed in, the unions feel comfy too. you cant go wrong with either. but from wat i saw, i liked the 390s better.
just my 2 cents