Tbh, despite the SG2 having 256Mb more RAM, im still gonna say the Sensation is the one to go for. Assuming the GPU in the sensation is comparable to Tegra2...
HTCs have always had better mod communities, and i know if i got the SG2 that at some point im gonna go 'hmm, wouldnt mind running android XXX now'. And with a SG2 i either wont be able to, it'll be a pain to do, or my ROM choices will be limited. Where as with the Sensation i'll have tons of choices & tons of ways to do it!
It'll be interesting to see the SIM free price..
On a different note, is the Desire S their 'next gen' 3.7" phone and the Sensation their 4"+ one?
As thats the only other thing that bothers me about the Sensation and SG2, is the fact that i prefer the 'size' of my Desire tbh.
Isn't the Incredible S quite harshly locked, eg to the point that over a month later it's still not fully unlocked/rooted (just from a quick google, could well be wrong)
If so that would imply the same will happen with the Sensation, SGS2 on the other hand I believe is using Kies/Odin again and should be if anything easier?
I dunno, could be completely wrong, but I think basing a decision now on possible custom rom availability down the line could be risky, best option in that case would be to wait until each has been out for a few days/weeks/months and see what one has the best support at that point...