Im not sure about how it will do on the 3g but on my first gen it scratched my chrome bad. I didnt believe all the reviews but they were true. They were tiny but noticeable to me.
Do you carry around sand in your pockets? I've been using the white slider for a week or so now and the chrome on my phone is not scratched at all. I have removed my case a few times and taking it off/putting it on hasn't scratched the chrome, either.