Writing Sample for Graduate School When submitting a writing sample for a graduate school application, what exactly are review boards looking at?

I ask, because when looking at my own writing I find it is difficult to judge beyond the grade I was given and when I can judge, there are different strengths and weaknesses.

For example, writing wise (clarity, concision, etc.), my strongest writing sample is on a topic that was written for a course in my field (Art History) but is still relatively tangential to the field itself (about visuality rather than Art History per se). That being said, topic wise, I have a paper that was written on the same topic as my fourth years honours thesis (still a work in progress) that speaks to the areas that I want to continue studying. It is still a good piece of work that received an A grade, but I personally feel that the writing and organization is weaker.

Which is better to send as a writing sample? The more on topic, but weaker piece, or the stronger tangentially-related piece.
I am only familiar with the humanities through my wife. When she reads writing samples for admissions she seems to talk about the quality of the research, organisation, and clarity. She doesn't seem to care about topic so much. It seems many Art Historians change their topic area early on during the PhD process in the US.