i love my sz520, altough i would not recommend the old ones due to the lack of inlays, it is really easy to get lost on that guitar. the 320 and 720 have
fret markers. aside from that, the 520 is a killer axe. mine is an original model, but with duncan alnico 2 pro and jb. crazy souped up
les paul tones. to answer your question: spec-wise, they are identical aside from adornment options.
rich