Are there any sonic differences between a glass and metal slide for electric guitar? Is there any particular reason why one would choose a glass slide over a metal slide, or is it simply just a preference over what feels best in the hand?
Assuming their surface finished quality are comparative indistinguishable, what then contribute to the tonal difference (or sonic differences?) would be the weight and density of the slides material. Weight contributes to the how well the slide is in contact with the string (mellow or sharper tone). Density contributes to sustain.
There are different grades/types of glass and metal. But in general glass sound more mellow and metal (usually brass) sound sharper. And besides glass and metal, there is also ceramic slides which also gives a different tone and feel. It really depends on personal preference.
See...It tends to be rather subjective....And likely dependent on the guitar as well.
I use a variety. Glass, brass, steel, copper.... To me, glass sounds bright and somewhat sharp. Brass is slightly mellower. Copper, being softer, even more so. I'm playing acoustic instruments...resonator guitars and also cigar-box guitars.