you can't go wrong with either phone. We tend to get caught up in the details and popular opinions. They are both really good.
A note about lightness of Samsung phones: I believe this is a cultural issue that Samsung needs to address. In the Asia markets, lightness of a device is VERY highly valued; whereas in Western culture, a gadget that feels light gives the impression of low quality.
Samsung has a huge market in its home territory where lighter is better. And they've managed to make the lightest devices out there.
Too bad over here in the US (and elsewhere), a lot of people complain about the lightness and plasticky feel. If you can help it, try not to judge the phone on weight. I definitely prefer more heft to my phones, but in no way do I consider the Samsungs inferior because they are light.
I tend to be a display junkie, and I much prefer the Sensation's screen over the S2's. This is totally personal preference, so there's no right or wrong. SAMOLED to me goes overkill on color saturation, which makes things took artificial to me. Some people love that aspect because they feel it makes the screen pop.
I'm in love with pixel density. Higher the better, and I have discerning eyes. To me, even iPhone's 326ppi could be better. I still see individual pixels. Sensation has the higher ppi compared to the S2. The screen is also brighter than the S2 (in terms of luminance). But the S2 produces blacker blacks, earning it the higher contrast ratio - divide-by-zero effect.
Sense is way more than pretty look and feel. HTC gets it right and gives their Sense apps a consistent interface. For people who have never used any Android phone, Sense scores highest in terms of intuitiveness (I'll have to dig up the study). Personally, I think Sense is decent, but in some ways, it restricts some functionality that other apps provide. So I do end up replacing parts of Sense. Not a fan of Touchwiz. Some parts of it are well thought out, but it's not as consistent overall as Sense.
Samsung tends to shine on specs, almost always beating everyone else out. But in actual use, there are some really basic stuff that they can't get right, no matter the model. I'll leave it up to you to research the common problems of the S2. At the same time, HTC is also known for having problems. You may have heard of the security holes recently found in some 4G models, and in their spyware (CarrierIQ).
So yeah, advantages and disadvantages to both phones. But both are Android superphones; very capable. I have no issues recommending either. It's mostly up to you to figure out what features are important to you.
