I have used both glass and plastic. Giottos (glass) and BestSkinsEver (plastic). Bottomline, the glass is by far the best looking in terms of optical quality. Glare is the same as the LCD itself. However, it is relatively expensive, and the tape that is used is just around the outside, so if it is not done properly or evenly, dust will get in. Being glass, it will scratch, but at least it is disposable. But scratches on a $30 cover can make you just as angry.
FWIW, the LCD already has a protective glass cover that is replaceable. However, I do not think it is replaceable by consumers, and must be sent to your camera maker. Just know that there is already glass covering the LCD.
The plastic does not have the same clarity, but unless you are pixel peeping on the LCD, you won't notice. Glare is gone completely. The clarity depends on the care with which it is applied: it must be very clean and smoothed over well. The plastic does not scratch, and at around $6, its easy to replace. The 'glue' is over the whole surface though some use attach via static or hydrostatic methods (no glue). I have the plastic on my smartphone, iPad and 40D LCD and have been happy. It removes easily, though you can not reuse the plastic if you remove it, as it stretches in removing it.
My recommendation: get the plastic, give it a try. If you hate the clarity, go for the glass.