Galaxy benchtests faster than the sensation but in actual use there is VERY little in it (certainly not as much as the numbers would suggest). Mate of mine has the S2, i played with it and it convinced me to buy the sensation. Have a play with both and see which you prefer, tbh I don't think there is a clear winner between the two, I reckon with the build quality of the s2 I would have probably destroyed it within a month.
*LCD vs. SuperAMOLED screen (the S2 screen is just simply amazing)
*less battery life (tech radars verdict: Well, it's going to happen again with the HTC Sensation, as the battery life, to us, seems woeful and not up to scratch. We're talking dead by 8PM with no push email, minimal Wi-Fi and about 30 mins of music and video at the most.)
*grip of death on wifi
*no swype (quite possibly the best invention on smartphone...ever, I have it on my Motorola Atrix and it is just amazing)
*SenseUI gash in comparison to TouchWiz (personal opinion but Sense is trying style over substance but fails)
*less RAM (768MB vs 1GB, unlike the CPU extra RAM really helps on apps, the Atrix has 1GB RAM and I can multitask with loads of apps open)
*
vastly less internal storage (1GB vs. 16GB on the S2)
not much to discern between the two eh
OP once you look at the screen on the S2 you will walk out the shop with it. I actually have stop myself from looking at my work mate's S2 because it completely ruins my otherwise very positive feelings towards the screen on my Atrix