Diamonds may be harder than glass but when bonded to a drill bit, the diamond substance is only as strong as the adhesive that holds it in place. Diamond bits tend to be very expensive and require a lubricant when drilling on glass. Both the lubricant and diamond dust can contaminate the repair and interfere with the overall optical quality of the repair. Shape of the cutting edge can make a big difference in your choice of drill bit. Tapered and fluted bits are a better choice if you are looking for a cleaner geometry of the drilled hole.
Tapered bits are ideal for creating mini bulls-eyes.
The best rule of thumb to remember is that the drill bit you choose should be the best one that fits your individual drilling technique and gives you the best repair.