Coming from the original Desire (which I still use for work), both the Sensation and the SGSII are huge improvements. I rooted the Desire to get A2SD+ after getting fed up of low storage warnings. I have 400mb of apps, so it makes no difference whether my new phone has 1gb, 16gb or 100gb.
I know plenty of people like the SGSII because it's thinner and lighter. I actually thought this was a point
against the SGSII - there's something nice about a handset with a little weight and a solid feel.