I beleive many posters are comparing two different aspects of the devices, the build quality vs the durability.
The Samsung GS II is obviously more durable since it has moving parts(not everything is snugly fitted) and to some extent has protection by the use of plastic in construction, the iPhone 4S on the other hand is straight glass on glass with a steel band around it so obviously dropping it will probably break it.
On the other hand there is the question of build quality which specifically addresses how well the device is put together and the materials used in it's construction. In this category I have yet to see a phone match the iPhone 4/4S. Like I have mentioned before, I kept a Motorola Atrix 4G for six months, it was a great device but after using it for a few months the parts started creaking under pressure when it was in my pocket or when I tried to press the unlock button. This doesn't mean it wasn't durable if I drop it in comparison to my iPhone 4S, the Atrix will probably walk away with less damage but what it did prove was that the build quality wasn't as good as the iPhone 4 I had for 11 months and had absolutely no creaking or bending of parts.
This is my personal opinion and perception of build quality vs durability. I think both devices are great and require care from the user but the iPhone 4/4S is more fragile even though in my opinion it's build quality is better.
EDIT: Would also like to mention to the poster above who was talking about cases, you can preserve the beauty of the iPhone if you buy a bumper and add a screen protector, I think it makes a huge difference because I dropped my iPhone 4 many times on concrete, tiles, mats, floors etc. but I never got a crack or ding on it. Just thought I would add my $.02 on that.