You are pretty much spot on. There is the rack, bean, and floor models.
The bean ones are meant to be sat on a desktop, plugged into the computer or whatever you can and for use with headphones. The rack ones would either be sat in a studio rack, a live rack, or again on a desktop, but they have a lot more connectivity options (XLR, optical etc). The floor ones are built for live use. They do have more connectivity options than the bean, but the form factor is not particularly conducive to being sat on a desktop.
I've got the HD500 but I tend to use it mostly for effects and as a MIDI controller for my live rig. I have done a little bit with it connected up to my recording rig but I can always get better tones (lead and rhythm anyway) with amps and impulses. The tones in the thing are pretty good though, they are workable and with impulses rather than the inbuilt cab simulations they are pretty cool.
Otherwise the internals are mostly the same. I think the HD300 and 400 have some limitations, but not enough to worry about.