might help you make up your mind a little. I personally have the incase slider and love it. It fits perfect and stays on very well. I would go with the slider if I were you.
IvySkin Quattro case is simply the best case. The only one with Glass screen guard and hard shell. I've had mine for over 4 months now and I think it's the only case that complements the iPhone's design. Read the reviews and decide for yourself! iPhone 3GS uses the same case as the iPhone 3G which I have currently. This is where I bought mine: CoverCase
Good luck!