Higher
Glass is smoother, harder, and more dense than tin. So the glass rub surface generates a higher pitch than a plated tin metal rub surface.

Solid
The glass rub surface produces a more "solid" tone too. Kinda like the difference between rattling solid natural bones v rib bones (which resonate because of the usually hollow marrow core). All our vintage pail-size metal rub surface washboards have back panels that form a resonating chamber. The glass washboards are designed with no back panels, thus no resonating chamber.

Tighter Buzz
Scraping the glass rub surface generates a tighter buzz sound than tin. That's because the glass corrugation waves are closer together than a traditional tin rub surface. How close? The pail-size Glass King corrugation waves are just 1/8" (3 mm) apart. The corrugation waves on a typical pail-size tin rub surface are around 7/16" (11 mm) apart. That's a ratio of 3-1/2 to 1. So you get more than three times the ribbets per scratch.

Good
Might show scuffs, scratches, minor cracks in wood. Ink might be faded or missing. Might show soap residue or other slight imperfections. No significant cracks in wood frame. No chips, cracks, or breaks in glass rubbing surface. No substantial scratches (i.e. washboard was not a musical instrument in former life). No missing parts. Framework is sturdy. Washboard has been around the block a few times but is eminently playable.

Better
Overall better condition.

Near Mint
Light wear. Hardly used. Little or no soap residue. Ink color and print quality usually good, but not always.

Mint
Appears unused. Ink color and print quality usually good, but not always. Belongs in a museum. Don't know why you would want to scrape up a fine antique relic like this to play music on it though.