I prefer plastic. I have a case that is very thin and slides into my pocket no problem. The rubber cases tend to stick to my pockets. Maybe this is a personal preference, but I like the plastic ones the best.
I'm all about the plastic cases. I also like the metal one's as well like the Element cases you can buy. They don't get stuck in your pockets when trying to pull them out either whereas a plastic/metal case would slide in and out quickly and easily.
I prefer hard plastic. My main case is a Seidio Innocase II, but I also have a cheap knockoff of an iSkin Solo that I really like. It's tecnically rubber but doesn't really feel like it in use. I like that one for a little extra protection when I'm camping or at the beach.
I've been going mostly case-less for a while now, using just a skin for the back of my iPhone and an anti-glare screen protector. When I do use a case, I use an iSkin solo or a Paul Frank silicone case since they work with the skin on my iPhone easily. I used to prefer an Incase Slider, but even though it's a thin case and works exactly as I'd want a plastic case to work, it makes the iPhone more bulky than I want to put in my jeans pocket.
I just purchased both executive cases in chrome ( goes well with my white iPhone) from casecrown.com.
I love my slider, but these cases ( the faux leather and the polycarbonate) look really nice, and they are CHEAP. both cases together are cheaper than one slider- and that includes $4.95 priority USPS shipping.
Because I don't like putting my phone in my pockets, (besides not being good for the phone, I've never liked anything in them), I guess I like them both. I am actually using both of them right now, (Otterbox Defender). just strap it in its holster, and voila!
I alternate between the two. I love my Incase Sliders for the summer. In the winter I like to use my Switcheasy Colors case because i need a good grip when its cold out. When I get to work I change into an Incase Slider case sometimes not always, depending on the color I am wearing. But before I leave i change back into the Switcheasy or a Belkin that I haved that I like.
I like my new Otterbox Commuter TL. It offers the best of both worlds, rubber around the case of the iPhone and a plastic strap on the back to help get it in/out of pockets. While I have yet to drop the phone carrying it in the Otterbox, it sure feels like it would survive a fall from my top pocket with ease. Hopefully, I will never have to find out