Single, of course. The math takes up a lot of space horizontally, and in two-column mode you need to set some of those math construct so that they cover both columns. It makes the style look less elegant, in my opinion, although I have seen several examples where it turned out fine. I am sure that double-column saves space for the editors - they can cram in more stuff in one page - but it just isn't as elegant.
Double-Column Papers squeezes some long mathematical equations or inequalities but presents lesser number of pages. Single-Page Papers makes work more clearer and contains the long mathematical equations or inequalities much better and in my view should be preferred.