Help, I can't decide! Both look fab - DH has just got the Sensation and it is nice but very big compared to what I am used to (Nokia 5800 Xpress Music, which is a delightful little phone and does everything I want it to, just not quite as well as one with better spec would ).
Important features are the camera, calendar, web browsing (mainly WiFi at home though), GPS/maps is handy and easy texting. Ideally quite robust as well (I don't like the Sensation's exposed USB port, it's just asking to get clogged up with Playdough and dog hairs).
Put it this way The galaxy is great, the best phone I've had but it's also the worst It freezes like mad, and the battery doesn't even last a whole day. I'm currently on my 4th one, lucky mines insured otherwise i don't know what I'd do
Tough choice. The SGS2 is arguably the technically superior phone. The AMOLED screen is to die for and is the fastest phone there is.
However I found it to be a bit plasticky and the thinness of it made me doubt it would survive a drop.
The Sensation looks nicer, feels sturdier and I love HTC Sense UI. Just little things like the way the screens can spin and the newer lockscreen is awesome. OK it's not as fast as the SGS2 but it's still like greased lightning compared to other phones so everything will work.
The screen size was a concern of mine at first but having used it, i much prefer it to the size of the iphone. The extra size really makes a big difference if you watch movies and surf the web. The GF likes it because it's easier for her to type with long nails.
The Sensation is pretty reliable. It was very buggy at first but two updates later and it's fine. Battery life is as shocking as any other smartphone and will have you reaching for a charger each day. It's not great but it's not the worst. To be fair they're all like that as you tend to use them a lot too.
To be fair though given your requirements and concerns over screen size, you may want to also look at the HTC Desire S which has a more traditional screen size.
All I can say is that I used to have a HTC Sensation. Now all I can do is lust after it after my GF adopted it and refuses to give it back.