I had a Defender for my 4. It was a thick brick that constantly required adjusting when I pulled it out of my pocket (the silicone outer was too loose). I had no problems with the built in screen (no rainbows) and I liked how you can use any headphones or power cords as Otterbox accounted for different sized plugs.
I'm now using the Lifeproof case for my 4s. This is my second one as I had some problems with the first one that I bought (see my previous post on this thread). The second case is perfect. It's not as thick as the Defender but it is taller. You need to use the OEM apple cord or else you have to buy an adapter that fits (I had one for another case, I think mine was a sendstation slim one). The case comes with a headphone adapter that you have to screw in, which I find is a pain in the ass sometimes when I need to use it. I find that the Lifeproof case is a much more pocket friendly case that the Defender as it is not as grippy as the Otterbox and it is thinner.
In terms of protection I found both pretty protective when it comes to drops. The Defender is probably a little better in drop protection (it is thicker), but not as dust protective (in fact, I find the Otterbox is a dust magnet). I've dropped my Lifeproof a few times at around 5 ft (by accident) with no problems. The first case I had (the one I returned with some issues) had a slight dent in the corner after a 5ft drop directly on the corner. I re-tested the waterproofness of the case and it was fine. My second case (the perfect one) I've dropped when showing some pics to a friend and it bounced around when hitting the floor, no damage whatsoever to the case or phone.
If you are used to the screen on the Defender, you should have no problems adapting to the screen on the Lifeproof. I found that the Lifeproof screen seems more fingerprint resistant than the Defender, but that could be because I can just simply rinse the Lifeproof under running water to clean it.
It's really surprising how much protection you get from the Lifeproof considering how thin the case is. It really puts the Defender to shame in my opinion. This being said, I would buy this in Best Buy so that you can return it if you end up with a bad one (like I did), and from some accounts their customer service is not up to par (I know Otterbox's CS is pretty good).