It's a tough call because there is no right answer.
I've had the hard shell with the Incase Slider. I can't tell you how many times that I have dropped my 3G in the last 6 months, at least twenty. And that is from 3-4 feet in the air onto concrete and asphalt. Still works perfect, though the case looks mangled.
In my opinion, the Incase slider has done a fantastic job at protecting my phone from physical blemish. Is the chrome bezel scratched, yes, slightly. But, it has done it's job in protecting my phone from looking like crap.
Right now I'm rocking the iSkin solo, aka the softy. Funny thing. I don't find myself dropping the phone as much because it's got more grip than my gunmetal slider.
My opinion so far.
The slider will protect from blemishes on impact, but it will definitely not dampen the impact.
My next case will also be the Griffin Reveal, that thing looks sweet!!