The main difference that I'm aware of is the GX-300 has an Optical Eye Sensor for automatically detecting crop marks on printed vinyl.
The CX-300 (1 generation older) has a crop mark detection tool that you use to manually align the crop marks.
I had a CX-300 and it worked flawlessly. Are you pairing this with a printer or a stand-alone cutter? If it's just a cutter, save the money and go with a used CX-300.