If the Galaxy SII and the Sensation are really close enough to each other that you can't form a strong preference, then I'd say declare it a tie, and then decide which service provider you'd rather use.
Since the Sensation is on Tmobile, you may also want to take a look at the G2X. Its supposed to be a strong contender too.
You really need to take a look at the phones in person, IMHO. Its not just about hardware and tech stats. The user interface is going to make a HUGE impact on how you use and enjoy the phone. EX: The Sensation uses sense 3 or something like that. So, its running android underneath, but its presented to you with this whole custom set up that feels totally different. (its supposed ot be pretty slick) The G2X on the other hand is running stock android software, without a skin over it, so its going to look and feel sort of like the original G1 layout. Kind of boring, and not ass gee whiz flashy as the Sensation, on the other hand, being stock and unlocked allows you to root it and customize it if you are the type that likes to tinker with your software.
You best bet for now is to just google "sensation vs " and see what comes up, then watch the videos. Heres a start