The otterbox is for the smaller AT&T version. It won't work on the sprint phone.
I use the Seidio Convert.
Seidio CONVERT Combo for Samsung Epic 4G Touch - Android Holsters - Android and Me
It's a fantastic case imo. 2 piece shell goes over the phone, then the rubber followed by the hard exoskeleton. The exoskeleton shell wraps around the corners helping to minimize damage if you drop the phone on it's corners.
It locks securely into the clip holder, I have bumped into things, pressed into it and the phone is still in perfect condition.
The one negative is the pesky rubber camera flap that you have to pull on to use the camera, it protects the camera, but it can be a pain to pull it open.
If you do decide to get the case, or any case for that matter, check amazon. They were much cheaper for this case than the actual seidio website.