GS500 has larger ccds, which would give you better low light video, probably better video all together.
GS500 has a focus ring.
GS500 has a 12x optical zoom. GS300 only has 10x.
There is probably more. It's just a much better camera.

I don't think either of them record in a real progressive mode. They may have a fake mode, but I'm not sure. Is that a feature that is really important to you??