Two roads diverged in a yellow wood
And sorry I could not travel both...
Now that I've spent a considerable amount of time on both Coursera and Udacity, the difference in UI design really stands out to me.
For an imperfect analogy: Coursera is like Windows, whereas Udacity is like OS X.
On Coursera, a course website is comprised of many sections, full of information like schedules and logistics. Course videos can be viewed online, but also downloaded easily, often with accompanying written and typed notes provided in a convenient format. The Coursera experience feels like a natural extension of a brick-and-mortar classroom experience; indeed, some universities offer rudimentary sites of the same nature.
Udacity, on the other hand, provides a single, streamlined interface, integrating course videos and quizzes. Lecture notes are provided in the form of a Wiki, auto-generated from subtitles and improved by the community at large. Udacity provides some simple FAQs, but really there's little to understand in terms of course logistics. You simply complete what you want, or can, at your own pace; there's not much emphasis on grades.
Perhaps the largest difference I've noticed so far is the assignment submission process. On Udacity, everything I've done is via the rudimentary Python IDE provided inline: complete the code and press "Test Run" or "Submit" for evaluation. This is very simple and easy to use.
Simplicity, of course, can also be restrictive. On Coursera, an Algorithms assignment might just expect some numerical values, so the student can write in any language and submit results via the web GUI. For classes that require additional software, like Octave/MatLab for Machine Learning, scripts are provided so that assignments may be submitted directly from the software environment in question (e.g. just call "submit" in Octave).
Although I have a slight preference for the Coursera format (mainly for the ability to review material easily), maybe it's not really about what's better or worse. What we see is the development of a nascent ecosystem, in which initial design decisions can reflect--and maybe also reinforce--something like a genomic difference between types of MOOCs. It's sort of surprising to think about how design, which is ostensibly cosmetic, reflects a deeper identity. But I guess this a question of the genre "turtles all the way down"...
