Is glass better than silicone?

Not in any quantifiable way. They're both very body safe, non-porous, phthalate free, hypoallergenic, easy to clean, easy to sanitize, shareable between partners and orifices. Neither material off-gasses any nasty chemicals or has funny smells. But many people PREFER glass over silicone, though plenty prefer silicone to glass. It's not that it's "better" but it's a preference. Silicone is usually soft and flexible. Even silicone skin around a firm plastic toy usually still has some give and softness. Glass, on the contrary, is completely rigid, which can be a benefit or a drawback, depending on your personal preference Glass tends to feel larger than a silicone toy of the same size because it has no give. Glass usually has even less drag than the best, slipperiest silicone and can offer textures that aren't often found in silicone. Glass also has the benefit of retaining temperatures, hot and cold, for temperature play. Glass is very heavy while silicone is usually lighter, again, may be perk or disadvantage depending on personal opinion. Glass also can't be used for hard thrusting because it can cause bruising if thrust too roughly, but the rigidity of glass makes it ideal for some elusive g-spots.
So it really depends on preference and your intended use. It's not actually "better" overall, but it's not worse either. less