dun ever swap sensation for galaxy s2. it is just like downgrading.

1) solid and premium aluminum unibody vs. cheap flimsy plastic
2) superbly gorgeous HTC Sense and its social features vs. iOS copycat Touchwiz

surely u will regret once the hype has gone down and u get fed up of flashing and reflashing ur ROM.

Firstly, ask yourself what you'd expect the phone to do?
Are you the geeky type who loves to experiment wt gadgets i.e. constantly customize the themes, changing ROMs, achieve orgasm when u improve ur benchmark scores?
if yes, then sgs2 is the better choice.

but shld u want simplicity, smooth and premium experience out of the box, with tons of functionality, then its iphone4 (even though it is 1 year old model).

anything in between then the htc sensation may do well.

also, if u cannot imagine urslf paying up to 2k for a flimsy and plasticy phone, sgs2 is not for you.
bcos of its thin material, u can feel the phone heats up very fast.