The Gloss finish is shiny and gives a classy and sophisticated look. The Matte finish is less reflective and gives an arty look.

The choice between gloss and matte basically boils down to personal preference as some prefer a classy gloss finish while others prefer an arty matte finish.

There is no difference in the material strength or quality between the matte and high gloss Skins.

There is no hard and fast rule. For example, the iPhone 4 is shiny and a 'gloss' Skin looks good on it. Likewise, an iPhone 4 can rock a 'matte' Skin equally well: http://istyl.es/gXkwvm. The choice is completely up to you!