I have the original slant darning foot and the 500 has much less fussiness with tension issues while FMQing. Still not my favorites because of the drop in bobbin assembly, but I was surprised that the 500 seems to be an all around better model, IMO. And I'm not just comparing one machine to one machine, as I said I've had probably 4, 401's in my hands.
Yes, that hinge is the weak spot for sure!