I've also been an ARC instructor for some time (trained at the same location, and likely same instructor). It's my experience that ARC training has always had strong programs for lay responders and the general public, and at least around here, ARC uses these health and safety courses to pay for all of their disaster response activities. That means there are fees for everything. Cards are $12, books are $15 each, mannequin rentals are expensive, instructor manuals, DVDs, etc all more $$.
AHA, OTOH, has always had very strong professional rescuer (or healthcare provider) courses (BCLS, ACLS, PALS, etc), and are the leading force with ILCOR in setting new standards (as ARC follows to a certain extent).
This is all changing, it appears. When I did my last instructor update with ARC, it appeared they were changing their business model. Instructor materials are available, online, for free, the course video is set up as a PPT, and free as well. They no longer mandate students each have a coursebook, but make it available online, for free, with ample opportunities for skill and knowledge refreshers over the course of the certification period. All of the materials are of course available in physical form, and they maintain that every student should have access to the materials (in some form), but have removed a lot of paper from the classroom.
New, and different certification authorities have popped up, and are looking more and more attractive to instructors and educational institutions. ECSI (the same company/organization that publishes the AAOS textbooks) offers an attractive suite of materials, for both lay- and professional-responders, in addition to many other courses. They give instructors (or educational institutions) their own cards to print, and eliminate costly overhead.
I still maintain AHA and ARC Instructor credentials (and reciprocity to ECSI), but haven't taught an ARC class in a few months (because I've been using ECSI or AHA's heart saver curriculum). I'm not yet sure what I think of their new business model, but I may settle in to it.
My advice to you is this: If you expect, or anticipate teaching friends and colleagues as healthcare providers, you should go directly for AHA instructor rating. The AHA course is shorter, yet more comprehensive, and I think better prepares you for your role not as a classroom lecturer, but a facilitator, supporting the video, which is what all of the certificating authorities have led instructors to become.
If you anticipate teaching lay responders or first aid classes, consider ARC, but then reciprocity to ECSI. You'll get a taste for the international influences of ARC, but also have a more practical credential.
I hope this helps, feel free to contact my by PM with more questions.