My kids have taken classes at AoPS Academy, so they've experienced both the books and the online platform together. Both have their pros and cons. My kids loved having the physical copies of the textbooks. Yes, they are online, too, but it was easier for them to curl up in bed or on the couch with the books.
For practice, both the workbooks and the online problems are great. But, the workbooks will likely take more work on your end to help them check answers, read hints, or try to solve the problems. They should be able to work more independently with the online platform.
