It depends on the question you are trying to ask. If your question is an algorithms question, Stats or SciComp would be good fits. If you want a statistics perspective on PCA, ask Stats. If you want a more general computational science perspective on PCA, ask SciComp. Sometimes, SciComp users are also willing to help with debugging; I am not sure about Stats, since I don't spend much time there.
StackOverflow, as far as I can tell, is mostly about programming. There are people who answer math questions over there, but the site isn't devoted to math algorithms; it's devoted to programming and software development. So if you want a programming or software development perspective on PCA, ask StackOverflow.