the S2 has a plastic outer casing around the screen so there is a much higher likelyhood of the touch screen shattering or getting cracks as a result of a accidental drop.
My HTC Design is a tank. I've dropped it twice back when I used to work at Wal-Mart and this was on hard concrete from over 5 feet with the second drop caused the battery to fall out. But put it back together and rebooted and it just shrugged it off.
Oh and I did not and still don't have any case protector or screen protector on this phone and only scratch on it after all this time is a hairline scratch at the very top that you would actually have to angle to the phone in a bright light just to see it.
The HTC Design has an aluminum or some other type of metal as the frame holding the phone together and the screen area is also held in place by the metalic part of the frame. So of coarse, the HTC will be much more sturdy.
Also, after going to my sister's piano recital, I used both my mom's S2 and my phone to record the event on video. The first thing I noticed is that the S2 has white balance issues and appears to have a smaller field of view and is also grainy despite recording at the max resolution of 1080p that it advertised in the specs. Perhaps it's Samsung's crappy camera app that might be at fault for it. (since both were on default auto settings in terms of auto focus and white balance/scene type)
My Design's video only goes to 1280x720, but it has a much easier time coping with fluorescent lighting and isn't grainy at all.
However the only con I've noticed with the Design is that it's got a terrible antenna design. Why the hell is half the antenna placed into the battery door? The result is sh**y 4G reception.