While all of them are good and have provided good courses for us students, I would like to give a brief comparison note between the 2 giants in the current market - Coursera and Udacity..
The Forums - Often good discussion among peers can result in better learning as the approach to problems by different individuals often vary.
In this respect both the sites have good forums , but Udacity forums make you feel more at home.
Coursera - The instructors sometimes don't maintain the forum and fail to take out time to respond to the user comments. The grouping of forum topics is more logical and finding topics are much easier.
Udacity - The instructors are genuinely involved and try to solve the problems faced by the students. Though they deploy tags to make finding of topics easier it is not well grouped and have random topics popping up here and there.
The Design - Well the objects and aids used to teach can often change our perspective and understanding about the topic.
Coursera - It is more or less like any video tutorial with concepts being written down on slides and then being explained by the teacher. Programming is often not taught and generally only the concepts are introduced and explained.
Udacity - The Instructors literally write down everything and work it out in front of you, hence all the doubts are instantly cleared as he explains every step at the time of writing itself. I think this is a major change that Coursera needs to implement in order to attract more students and teach well..
The Teaching- Well I dont think I need to stress the importance of a good teacher.
Every teacher actually has a unique style of teaching and may suit sets of people. We can't really compare the teachers as all are accomplished and the learning is also partially dependent on the students approach to the subject and how much it suits with the teacher's style.
In my experience the staff in both the sites are equally good.
Content Difficulty and Course Planning- This is pretty self explanatory, so I'll jump right into the comparison.
Well it maybe because of the more hands on approach but Udacity problems and programming assignments are far more easier than the ones offered at Coursera. In Coursera the Programming Assignments and Problem Sets often hold marks and thus a user has to go through all of the lectures before actually attempting them whereas in Udacity it is often the case that we go and view the lectures only when we get stuck. This model is being implemented in Coursera by introducing untimed assignments which have to be submitted only before the deadlines so as to facilitate more flexibility.
Grading - As we expect certificates, there has to be some grading procedures to evaluate how much a student has actually learnt. Getting the certificates in the both the sites are actually pretty easy thus reducing their value in the world, but naturally easy certificates means more joinees and hence more investor backings!!!.
Coursera - There are weekly assignments and problem sets in most of the courses and these require dedicated watching of the week's lectures to actually see them. They provide tracks which would separate the better from the good and also provide your achievements as a percentage though you rarely know how good this actually is as compared to your peers . The courses provide certificates for anyone securing more than 70/80% of the final score and this is quite easy in most of the subjects. There are of course exceptions.
Udacity - There grading is based on a final exam. One bad day and you can actually screw up your entire course performance. The grading is better as there are more levels of achievements. There are ones which say you have completed, accomplished with high/highest distinctions.
Money and People- With the introduction of newer universities Coursera has gained lots of Financial Backing but still lack the number of students as most of them are opting for Udacity. Udacity in turn are recruiting top students of the different courses they offer to make their forums more active so as to attract new students and investors.They hope that this will make up for the width that Coursera is offering.
Hope this helps and my advice would be if you have time try to do both..