Neither option will affect output, if you’re talking SPL. Wood actually only absorbs at certain frequencies, not the entire audio spectrum. That would be more of an issue for room acoustics - say, a wood-paneled walls vs. sheetrock – not really a consideration for this. However, another issue possibly more relevant (even if vaguely so) is that wood better absorbs vibration from the cabinet than glass does. That might be an issue (again, if at all) if the cabinet was sitting directly on the wood or glass shelf. However, the speaker will be largely decoupled from the shelf due to the feet it has.
Bottom line, acoustically speaking glass or wood isn’t going to make an audible difference, so go with what you like aesthetically. However, a wood shelf is better from a safety perspective, as you can secure it to the wall brackets, while a glass shelf will merely be sitting on the wall brackets and could be tipped.
Regards,
Wayne A. Pflughaupt