Feel like I've been in this thread before, but in the SGSII forum...
I own both, and I bought the Sensation after the SGSII. It had to do with dev support and build. HTC products feel like equipment: they're sturdy, metal and rubber - built to last. Samsung: not so much. More like lightweight, easy to slip out of the hand without a case.
The Tmo SGSII forum is a gotdamn mess. All kinds of kids crying in there and devs leaving (see thederekjay). Terrible that its really no ones fault since the dev's hands are tied and GB Touchwiz roms are all you're really going to get for now.
In here - the development scene has been buzzing for months. Off the charts with how much has gone on in here since this device was launched. Anywhere that rmk40 devs, I am sure to follow.
Either way, for me both phones are just waiting to eventually be sold before they're not valued as high, as I see the next best thing may be the One series.
Nexus 4 Unlocked, Rooted, and currently flashing like a perv on a jogging trail.
GSM GNex: Unlocked and Rooted (Sold)
Rom: Stock 4.2
Sensation: retired S-Off, Rooted, SuperCID, Unlocked, 32GB MicroSD
Rom: Virtuous Inquisition
Samsung Galaxy SII (SOLD): Rooted
Rom: Darkside Digital Warfare
Vision/G2 (SOLD): S-Off, Rooted
Rom: Andromadus v11