First impressions of 10 mins in store with both phones.
S2:
Incredibly light, truly incredible form factor
Extra 256mb ram gives it a little extra snap
Screen is a little garish in terms of contrast and not actually as bright either (could be the dimming software though)
Sensation:
Lovely resolution
Feels weighty- good or bad depending on what you prefer
Sensation software is great
Little less snap than s2 because of less ram
Basically, if the s2 came with the sensation’s screen you would have a winner.
I am erring towards S2 because it was SO light and SO thin and felt a little smoother than HTC.
Sensation software is really nice and the extra resolution on htc is special.
I would say that you'll find that the Sensation has plenty of memory. The problem lies in Sense; it's a very heavy interface indeed. Yes, it's great looking and works extremely well, but the performance is terrible.
Scrapped it on my DHD, though will be taking a look at Sense 3.0 ROMs when they are done.
To be honest, all of the speed of all of the phones relies completely on the software, not the hardware; take a look at how smooth the iPhone interface is, with its hardware that is slightly slower than most of the last-gen high end phones.