I can only speak for WGU.
Pro's... flat rate per semester, about $3600. Do as many courses as you wish in the 6-month semester for the one price. You can test out quickly on many of the courses. How quick? I finished several in a matter of days or less.
Not too many papers you have to write (depending on major)
pretty liberal on accepting transfer credits.
You can test out 24/7, you have to do a pre-test before the challenge test to complete, many times the pre-test is VERY similar to the challenge.
Thus far, all the instructors are quick to respond and to provide assistance.
Con's... IMHO, you have a mentor that is up your butt, you have to show progress on a regular basis, or they can pull the plug on you.
Not too much leeway on changing courses, the course plan for a major is not as flexible as I would like.
I self pay, I still have to deal with financial aid to get their blessing.
For the price, it is hard to beat. If you desire a Masters, the same format and about $6-800 more per 6 months.