A few key differences: the $350 model has a heated build platform, larger print envelope, but slower print speed. The heated build platform allows one to use a wider variety of materials---in my experience however, for these consumer printers it's better to stick to using PLA for anything but small objects, unless you're specifically designing for FDM. The steel frame should be an advantage (weight and stiffness to dampen vibrations) but the weights listed on the spec sheet show the $350 and $400 models to be pretty similar.
In the end, I don't know that there's much of a difference based on the spec sheets. It'll probably take someone with direct experience with these models (or the printers they're based on) to decide between them.