Processor wise, both the Sensation and the Galaxy S II feature dual-core processors clocked at 1.2 GHz each. But that doesn't mean that the two have matching speeds and performance. Sensation runs on the third generation Qualcomm Snapdragon chip, while Samsung's Exynos chip is currently the processor to beat. If we go by mere benchmarks, the Exynos chip in the Galaxy S II outruns the third generation Snapdragon processor by a fair margin. But that's possibly the only difference between the two and a very decisive one to be honest.
If you are looking for a phone merely for its usability and looks, then the HTC Sensation with its stylish unibody design and the typical HTC form factor is the best pick for you. Additionally, it sports a qHD resolution (540x960) screen. That being said, if you're looking for sheer performance on your device and don't pay too much heed to looks, then Galaxy S II is just the right device for you. We can't forget to mention the ever so remarkable Super AMOLED Plus display, which improves viewing experience considerably, and ultra slim form factor with a very light body. Both the handsets run on Android 2.3 Gingerbread.