In my opinion, the best protection is some kind of invisible adhesive protector (i.e. Zagg Invisible Shield). I personally like Phantomskinz, as I have used their products before and it was very good.
Honestly, I think any slider phone will be good with a hardcase (or softcase for that matter) because many times it gets in the way of the sliding mechanism. And specifically for the Epic, its just so big to begin with that I wouldnt want to add additional bulk. It's not TOO big, but I fear it might become so if a hardcase is used.