I've taken 8 of them so far. In general, I have found the iQuilt classes more advanced than Craftsy classes (yes, there are exceptions on both platforms). Also, the Craftsy classes are usually more project-oriented whereas iQuilt classes usually involve mini-projects that focus on skill-building (i.e., the skill that is being taught dictates what you are making).
I personally prefer iQuilt classes, though there aren't very many currently being offered (they add more every few months, it seems). My absolute favorite is the borders, bindings & finishes class taught by Bonnie K. Browning. It's hands-down the best class I've ever taken anywhere -- live or online. Because of a disability, I rely a lot on online education (Craftsy, iQuilt, National Quilters Circle, CreativeBug, YouTube). I have found the iQuilt classes to be exceptional quality. They offer promotions on a regular basis & AQS members get an additional discount on both regular & sale prices on classes & materials.