One thing I will add is that the only time I find the glossy paper to be a noticeable improvement over the matte paper is on any cover that uses a lot of black. I use glossy paper for my PS1 and PS2 covers because it looks a lot closer to the original PS1 and PS2 covers. For pretty much everything else, I use matte.
Fortunately, PS1 and PS2 covers can be printed out of letter sized paper if you print margins will allow for it. And letter sized glossy paper is super easy to find anywhere. Anything that requires legal sized paper, I go with the matte paper.
Also, while SNES covers use a lot of black, since 100% of those covers will be custom made and printed, you don't have to worry about matching them to originals. That's why I use glossy with PS1 and PS2. I only have a few of each of those games without original covers, and the matte paper sticks out like sore thumb when they are all sitting on a shelf.