My contract is just about up on HD2, and I've been in a similar predicament. I dual-booted my HD2 between WinMo and Android for a couple of months, before realising I spent no time in WinMo and 100% time in Android, so put Android on the NAND and never looked back.
I've been looking at the SGS2 for a couple of months, then couldn't decide between that and the Sensation. Fortunately, the bloke at work who annoys me got a Sensation, so that sorted the question for me and I have a SGS2 on order from e2save and it should be here tomorrow morning. £30 a month, on a 12 month contract, with a purchase price of £119 (which I got for my HD2) couldn't be beaten!
The only negative on the SGS2 compared to the Sensation is the screen resolution, but after seeing both screens side by side, I simply couldn't tell the resolution difference, but the SGS2 screen looked a lot more vibrant, and less slightly-washed-out than the HTC. Sense on Android is a take-it-or-leave it thing for me (on WinMo it was essential towards the end) so that doesn't bother me. I'm also looking forward to having a decent camera for the first time on a phone! The Sensation is the best camera HTC have made and my colleague still categorises it as "crap".
Incidentally, in case anyone thinks I'm down on HTC, here's my phone list since 2003: HTC Canary, HTC Typhoon, HTC Magician, HTC Hermes, HTC Kaiser, HTC Touch Pro, HTC StrTrk (£25 SIM free brand new spare phone = in pink!), HTC HD2, Samsung Omnia (got-it-free spare phone). I've been an HTC fan for a long time, it feels weird to buy a phone from a "mainstream" manufacturer like Samsung again!