I am doing Princeton Review's online course, and I would definitely recommend it. The lessons are based on my performance (i.e. weak spots get more attention), although I can do any of the drills anytime, but I really like having them customized. The online instructor chat is very cool and helps a LOT.
I haven't taken Kaplan, kinda doubt most people who'd take one online course would have taken two, but in general if you like one company's books/tests over another, you'd probably like their course better. I liked PR's book a lot better than Kaplan, so I went with them for the course.
Let me know if you have any specific questions and I'll try to help!