[jsyedidia]

I would not lump edX in with Coursera and Udacity. As noted in Wikipedia: "EdX differs from other MOOC providers, such as Coursera and Udacity, in that it is a nonprofit organization and runs on open-source software."

At first, it seemed to me like Coursera, Udacity and EdX offered similar quality courses. But at this point, I think that, on average, the EdX ones tend to be significantly better. And that difference does seem to me to be related to the fact that EdX is not a for-profit institution trying to extract money from you. Also keep in mind that the EdX systems were largely developed at MIT, and EdX was originally intended as the improved version of OCW.

[xiaoma]

I would. Just looking at CS, for example, on OCW you can get the any course needed for a degree (or even some graduate degrees). In contrast, on edX the vast majority of the courses are either introductory, sub-introductory (e.g. Computing: Art, Magic and Science) or an intro to <random MS tech>. Even when filtering down to "advanced" the top result is a freshman level course—Data Structures.

Furthermore OCW is rigorous. It includes actual homework assignments given and final exams from previous classes of MIT students. Coursera and edX are extremely focused on videos, which IMO are not the most time-efficient or thorough way to learn. The one area in which Coursera and edX shine is their automated program graders.

On the whole, I would say that if you want to learn math, CS or any hard science and you hope to actually pass a subject GRE, apply to grad school or build a truly strong foundation, then OCW wins hands down. If you want to pick up a new programming language or tool, especially one from Microsoft, then use edX (or Coursera).