Sophia.org
Sophia is widely considered students premier choice for completing ACE Credits. That is mostly because of the speed of courses (87% of my audience voted Sophia Learning as the fastest courses of any ACE platform), and the $99 a month price tag which is very affordable for ACE Courses.
Course Structure:
Sophia features a system that has challenges, milestones, and touchstones in their courses. Challenges and milestones feature problem-solving such as multiple choice questions. A touchstone usually involves some type of project or assignment.

It can be a little complex to understand, but once you're inside of the course and start working on it, it's self-explanatory.
User Interface:
Sophia does not offer a course preview for guests but their user interface is clean, simplistic, and very eye appealing. It is a light-theme but is attractive to the eyes.
Pricing Model:
Sophia is $99 a month, and that is for unlimited courses to my best knowledge.
Is Sophia proctored?
Sophia has no proctoring to our knowledge, however, sometimes you do have to wait for someone to grade your touchstone.
Average Course Length:
Anywhere from 1-7 days per course is entirely feasible.