SGS2 - King of andriod as of today
HTC Sensation - King of HTC as of today, not as good as SGS2

You're being biased. State your points as to why do you think the Sensation is not as good as the SGS2. Don't just answer without any valid points.
All android phones have the same selection of apps excluding QVGA reso-ed phones.

Hmm. Do you intend to use your phone as a source of music or are you intending to plug in headphones. If you intend to plug in headphones, I suggest you get a headamp cuz the stock sound quality on the phone is a major drawback compared to its predecessor. I won't go into detail cuz it might derail the purpose of the audiophile section but if you're interested in learning more feel free to drop me a PM.

Back to your question, your requirements are pretty general. So all I can say is this.

If you're going to customize(root/custom rom) your phone in the future, I'll say the SGS2 would be better for you.

If you intend to use your phone stock, then the Sensation. But as far as I know, the screen sensitivity issue and the sharpness issue is still not yet resolved. If it doesn't bother you, go ahead with the Sensation.