From a support perspective (and thas't what I do) both phones generate almost identical support calls from customers, both in quantity and issues.
The issues (and perceived faults) are, in the majority customer based issues not phone hardware or software issues. both phone are very solid performers and I would bet that dropping either of them onto a corner would produce the same faults.
As most customers, have no real idea about the quality or performance of there smart phone/lap top/ PC/ car/ TV, need I go on.
Then the issues here are mainly about what 1 person prefers over another, as despite the better performance of the GS2 the difference in real world usage is almost negiable. The difference is only noticable (imho) in bright sunlight, as HTC are behind the curve there.
I have the Sensation and choose it over the GS2, why, well I like HTC sense, the feel of their phones, the ability to flash custom Roms, my dislike of Samsung support, the poor quality of their support tools, the fact that on average software is not as good as HTC (look at the recent and ongoing issues with the Ace)
Overall as a user the HTC overall package has very few flaws, the Samsung just slightly more.
However to confirm, the GS2 the better phone.
I have very little IPhone experience on the 4s so can't add anything of value there
Users who are more knowledgeable tend to pick faults to justify their own decisions to or not to buy