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The hard plastic case that I bought for my iPod a couple of years ago ended up scratching up my iPod more than not having a case would have... I've since strayed away from any hard plastic cases.
 
I'd stay away from hard cases, and lean more towards the silicone or rubber cases since they won't scratch your iPod, and they will also offer a little protection from bumps, dings, and short drops.

A hard plastic case won't do much good if you drop the device.
 
supposedly the hard cases are supposed to be better. They don't scratch because everything is tightly packed together. A rubber case could trap sand or abrasive particles between the back of the touch and the case, thus rubbing would scratch the back. Also when you drop a hard case the case is suppose to take the impact, not the touch. I am not sure how valid this is cause I have never taken a physics course in my life.
 
As long as the clear case is designed to fit snugly on the touch, then it shouldn't scratch the screen.

I've been using a clear case for my nokia n95 8gb for 10 months now, and the phone still looks brand new with no scratches of any sort.
 
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