Below is a very brief comparison between Samsung Galaxy S2 and HTC Sensation.
I am a Samsung Galaxy S2 user. Despite its superior features, there are some drawbacks on this monster. One of the obvious disadvantages is the battery capacity. If you turn on the data connectivity for browsing and play games all day long on the Galaxy S2, its battery can only last for merely 7 hours or less. To extend some battery life, try using WiFi for internet connection. Besides that, changing to a larger capacity battery could extend the battery life to 10 hours or more.
Another problem with the Galaxy S2 is heat. Samsung had successfully shrink this monster down to less than a centimeter thick, but this also means that heat will be dissipated much easily to the external surface. You will feel a warm sensation if you turn on this beast to its maximum capacity for long.
Even though the Galaxy S2 has some major flaws on its design, its Super Amoled Plus screen is definitely the best screen technology among the smartphones. With much lower battery consumption, the Super Amoled Plus overtakes the LCD technology by its aiblity to produce a much vibrant and sharper image. If you do not catch what I mean, just drop by the Samsung store and compare the Amoled and LCD screen side-by-side. You will be amazed!