I have a 3GS and while I can't speak about the iP4 Commuter specifically, I can tell you that I LOVE the one on my 3GS. It's the perfect mix of protection and a slim design. The silicone Otterbox uses isn't a lint magnet like other silicone pieces I've had and with most of the out shell being plastic, it slides in and out of a pocket easily.
I also have a Candyshell for my 3GS and while I love it too, having to deal with separate headphone port and dock connector protectors can be a bit of a pain. Yet another thing I love about the Commuter...the built-in port protectors. I've also found that I have some trouble with the Candyshell with aftermarket headphone plugs...sometimes the hole is just too small for them to fit. I've yet to have that problem with the Commuter. Because it's silicone that surrounds the headphone port and not plastic (like the Candyshell), bigger headphone plugs will just squish the silicone out of it's way.
There's no doubting the Candyshell offers fantastic drop protection too. And, because it doesn't actually cover the edges of the glass on the screen, you won't have any problems with it messing up a screen protector. However, I've found first hand that the Commuter is just as good at drop protection and even though it does cover the edge of the screen, because the silicone is soft, it won't mess up a screen protector. I dropped my 3GS while pulling it out of my back pocket and it landed on the top right corner on the curb, bounced, flipped over a couple of times in the air, and landed screen down on the street. There wasn't a scratch on the phone or screen and the case (because it landed on the silicone on the corner) didn't have a mark on it.
One thing about the Commuter...the hole in the back that shows off the Apple logo on the back of the phone doesn't have any protection. It's wide open. On a 3GS, this is probably more of a worry than on the iP4 (plastic back vs. glass), but I cut down a clear screen protector and put it over the logo so the back wouldn't get scratched. This does the trick nicely and keeps any little scratches from appearing. Also, if you've got some sort of screen protector on the back of the iP4, the Commuter skin should fit over it just fine.
IMO, you can't go wrong with the Commuter.
*edit* I just realized that through all that, I didn't answer your last question...lol. The Candyshell does add a bit more bulk to the phone than the Commuter, but not too much. Both of them are slim enough to get in and out of a pocket easily.