Okay, counter-point: in my experience Udacity has the highest quality content on the toughest technical subjects. To me, it is evident that an enormous amount of work has gone into making the lectures and exercises effective.
Coursera has more courses, and the quality varies more but some of them are excellent. In any case, Coursera lectures come from university partners we are all familiar with, so people can decide if they like the Andrew Ng Machine Learning Class or Geoff Hinton on Neural Networks.
I've sampled courses from several and found the quality varying. Each will have one or more super high quality courses, then a swarm of forgettable ones. Which is only to say, don't rule out any particular source but instead consider the subject you are interested in, then do a little research (or sampling) to find the best offering wherever its hosted.
