the PC. There is more to it but that is the answer to your question.
EDIT WHY: Missed the why part. Pre-coffee, typing in bed post. Sorry. OK so why.... I am going to qualify this the fact that your question really is not answerable but I am answering your intent, not the question.
What I am answering is "If i could install Windows XP on a mini and have fully supported drivers for all hardware on the Mini, at the point of initial install which would be more responsive." You asked PC vs Mac. Ignoring the OS the question is unanswerable. Further there is not an equivalent PC to the Intel Mini at this time so there is not a pc with the same specs on market.
With fully supported drivers, blah, blah, XP is more responsive than OS X at least initially, and for some users this does not slow down. The reason for this is multi fold. First OS X abstracts the UI through OpenGL calls, this leads some users to feel that OS X is "sluggish" compared to Windows. Window resizing in particular shows this as an example. Next is that MS uses a better compiler for the OS than Apple currently uses. GCC has its strong points, but optimal it is not. Further on OS X far more superfluous visual effects are done which slows down the impression of speed.
I could go on but that is the jist of it. I still feel this is not a useful question but it is what you asked so I have done my best to answer.