The 4S does feel a little more solid than the S3 too, but I'm happy not to have the extra weight in my pocket.

I've also seen a lot of iPhones with big cracks in the glass in the back and I'm happy I can just swap out the plastic back of the S3 with a cheap replacement if necessary.

I've seen 3GS practically shatter from a drop of 3ft. I've seen S2 flying all across the room on the concrete floor and not a scratch.

I'm not trying to draw statistics here, it's just my experience that iPhones are more resistant to wear and tear, and Samsungs more resistant to somewhat more extreme shocks.

The battery coming out absorbs most of the impact energy. It is an excellent design, as you get user replaceable batteries as a side effect.

If the back plastic is anything like the S2 back you'll never need to swap it at all. That thing is surprisingly strong.

https://www.youtube.com/watch?v=etsxu0OMqUQ