Rubber beats silicone for keeping a mousepad flat on glass.

Yes, it matters. Rubber has more static friction on glass than silicone does. Silicone is slicker and tends to slide around, especially if your desk is even slightly glossy. Rubber grips better, stays put, and resists curling at the edges.

That said, neither will magically flatten a warped mousepad. If the pad itself is bent from packaging or heat, no base material fixes that. But on a clean glass surface, rubber is the safe bet. Silicone works fine on a textured desk, but glass is where it fails.

Skip the aesthetics. Get rubber.