Python got popular earlier - it was already gaining significant converts in the early part of this millennium, and the later years of the last one. Ruby only really took off when Rails started to get popular. But it has really taken off, in a way that I think sometimes makes the Python guys (who have been slowly and steadily gaining ground) a bit jealous.

They're both good languages with good communities though. Each has a few advantages and disadvantages, but they're similar enough that if you know one you should probably stick with it and be happy, and pick something farther from the tree as your next language (Erlang or Scala, say).