My wife and I have iPhone 4 in Otterbox Defender cases. They are good cases, but not perfect.
* very sturdy
* non-slip surface
* still susceptible to water intrusion
* integrated screen protector is cheezy
That said, I think the Defender protects the phone well. If you are interested in an Android phone, the Defender is also available for the Galaxy Nexus, which is probably one of the better choices for Android.
Note Case Mate makes a case called the Tank, which looks pretty good as well.
With my case, what I eventually did was pull out the integrated screen protector, and then put some putty around the edges and install a "normal" screen protector. It was kinda a pain, but I prefer it. My wife is still using the integrated screen protector.
For water intrusion, I carry a medium SealLine E-Case case in my EDC bag. If water gets serious, I can throw the phone in there. Note the pictures show a phone in the small case, but the pictures are misleading. The phone won't fit in there with a case installed!http://cascadedesigns.com/en/sealline/protective-cases/e-case/product
From my perspective, the applications are more available and better on the iPhone, but the Android phones are more "open". Also keep in mind the iPhone does not have a removable battery, so I keep a Tekkeon TekCharge rechargeable battery pack with me:http://www.amazon.com/gp/product/B004OC0122
And lastly, I have a Logitech UE 500vi earphone's with the built in mic., and I cut off the non-mic side so I can have it in and listen to music, or talk on the phone without losing the ability to hear what is going on, and not have the problems of getting the loose ear caught in things.