This is a link to a site where someone asked for feedback about AFTA. It is pretty interesting to hear what people say. It is all just opinion, but may be helpful to you to hear different opinions.
Since you have had no answers directly from people who have done more than one of these I will give you a couple of general thoughts that I think are worth while thinking about.
I have taken ACE, an online one whose name I forget (not AFTA), and a long time ago the written part of ACSM. I have also taken exams within the context of University classes. The online one was laughable, and I have since learned, but am not surprised by, not very well accepted in the practical world to which it belonged. The other two certification exams were hard. The university tests were hard, but we were very well prepared due to the semester of training. I think it is good to do a hard exam, as it shows one that one knows what one needs to know to work in that area.
Think about where you might wish to be employed and see what certs. they accept.
A proctered exam will have more credibility than an online one.
The training field is quite full. To compete you may find it helpful to have not the easiest, or the cheapest, but the most known and most respected, so figuring out what that is is helpful.
You are going to be looked at for the whole picture: education, experience, and certification.
Good luck with your exam.