During the test, I applied both matte and glossy screen protector (from my own online store, www.canaloo.com) on a iPad 3. The photos were taken with a iPhone 6 Plus. One thing to notice is that I applied the screen protectors fast and poorly, as I did not clean the iPad screen thoroughly and it won't make much of a difference in these tests.
Matte screen protectors are often called "anti-glare" screen protectors. This is the major reason why people would choose matte over glossy.
First, I took a photo under normal to strong lighting condition. The photo was taken indoor where reflection of surrounding objects is expected. As shown, the matte protector did a decent job blurring out the reflection, while the glossy protector reflected a like a mirror. I felt too much of eyestrain a result of prolonged use of a glossy screen. However, the reflection could be potentially reduced by changing the reading angle.
Next I took the iPad outdoor where the sunshine was blinding. In this situation, the matte still has an edge over the glossy in terms of glare reduction; however, the advantage is a very slight and it's a bit hard to read through both protectors. I think it's the direct sunlight that blinds you not that reflected off the screen.
There are two aspects of image quality, clarity and contrast. In this category, matte screen protectors typically suffer less contrast, less brightness, and grainy image. The photo below is taken directly above the iPad and under normal lighting condition. To help identify the difference, there are two viewports with zoomed view.
From the left viewport, you can see the matte screen protector introduces grainy feeling to the image. Contrast is definitely bit off as well, for the left image is just not as sharp as the right one. However, I don't find much of a difference in brightness. I believe this probably goes to the build quality of a matte screen protector.
Lastly, the glossy screen protector is seamlessly in terms of image quality.
Many of you might not know, a matte screen protector is also more resistive to scratches. I used my butcher knife to slide on the protectors in both strong and weak strength.
Both screen protectors did not survive the strong cuts; while for soft cuts, the matte screen protector is much more resilient.
A matte surface is just not as easy to get dirty as a plain (glossy) surface. In this test, I rubbed my normal human fingers (probably a bit oily) on both screen protectors.
The result is obvious. I can probably use the glossy screen protector as touch ID to unlock my iPhone.
Overall, matte screen protector wins in three out of four categories. Well, the result is expected. It very much depends on how you distribute the weight over these four categories and how you judge the scale of difference based on the photos above. Personally, I would choose glossy screen protector, since I usually work indoor and watching videos quite often. I do not want to compromise image quality, even if it's off by a little bit.
Go for matte screen protector if you: 1. Don't clean your screen frequently. 2. Often put your device with keys. 3. Prefer readability over image quality.
Go for glossy screen protector if you: 1. Mostly use your device indoor. 2. Really care about image quality (i.e. movie lover, photographer).