The Defender will give your phone more cushioning and the built-in screen protector (not part of the Symmetry series) will guard against scratches. If you're concerned about dust, sand, etc. getting into your ports, the Defender is a good choice because it will keep them covered.
On the flip side, the Symmetry is a good choice if you want a lighter, less bulky case, and it's easier to slide in and out of your pockets. It also keeps the headphone and USB ports open, which is nice if you're using them a lot. No built-in screen protector with this case, but you can purchase a screen guard separately if you're so inclined.
10 months ago