I have 4 galvo fiber's and 4 C02 gantry machines, no experience with a galvo C02.
However, I do a few hundred wine glasses a year on the C02 machines, and for ME at least, I've found I can only get decent, consistent results if I'm engraving from a halftone bitmap. My 2 Gravograph machines have wonderful photo engraving capabilites, and to do glass I just paint it green and tell the software it's a photo. 80 to 90% grays work too, but plain old RGB green gives me good results.
BUT, my Taiwan made Synrad equipped GCC Explorer, despite several built-into-the-software halftone options, I could never get it to engrave glass consistently.
That is, until I let my Universal 1-Touch photo program create a bitmap for me. BIG difference, much more consistent, much less 'big chunk' random glass fracturing.
I can't say enough good about 1-touch, with it I do most wood photo engraving in my Chinese Triumph now...
One of my favorites, converted by 1-touch and engraved in the Triumph:
From this,
pink floyd original.jpg
I got this--
pink floyd wood.jpg
For doing glass, I just had 1-Touch covert 90% gray to a bitmap, worked great! Nice thing about 1-Touch, you can try it for free, and the "trial" version is no different than the paid version.
Even creating a bitmap using Corel works pretty good, but for my GCC it likes 1-Touch better.