Well, I finally jumped ship, reluctantly.
I was offered too good a deal by Vodafone to not say no.
I deliberated long and hard over the Samsung Galaxy SII and the HTC Sensation.
I spent over an hour in the Vodafone shop testing both phones before deciding and then finally plumped for the Galaxy SII in spite of being a life long Samsung hater.
My reasons were:-
The Super Amoled screen of the SII is amazing in all lights. Very clear in daylight, the best I've seen.
The extra RAM on the SII gave me almost 3 times the free RAM than that of the Sensation after boot up.
The 2GB of internal memory on the SGII allows me to keep all my app's there with 1.5GB to spare not to mention the 16GB of phone memory (on my model) and the, up to, 32GB SDCard.
Excellent camera and speakers on the SII, better than the Sensation IMHO.
Since I'm a big Cyanogen Mod fan I knew that I would be rooting my phone immediately and so TouchWiz and Sense 3 did not come into it.
Bigger battery on the SII.
Having said all that, I've still got my trusty Nexus One and continue to crack flash it nightly.