What is the difference between glass bongs and silicone bongs?

Glass bongs are usually made with quality borosilicate glass while silicone bongs are made with food grade BPA free silicone. Glass is more heat resistant, and many people prefer the feel of glass as a texture. 
However, glass bongs are more fragile than silicone bongs. Silicone bongs are flexible, and can be rolled up, and easily stored for travel.