Glossy paper is shinier, looks like having high contrast and makes the color “pop” on the stickers. The extra shine works well if you are planning to load your sticker with a lot of colors and big graphics.
Matt paper has a more muted feel to it. If you plan to use light or pastel color scheme in your stickers or aim to have a lot of text on it, then you may choose to go with matt finish.
Having said that, it is all about the usage of your stickers, if you want your stickers to have more shine, then go with glossy finish, else matt finish works just fine!