From personal experience, both classes have fairly high skill ceilings.

AST: Knowledge of your arcana is important and how they work with other classes to get the most out of your cards. We have lots of fun CDs that can be used often and don't always have to be saved for "osht" moments. Our newer minor arcana gives us a bit more to do and helps a ton with top-decking or dead draws. Versatility means we work well with ANY healer, even other ASTs!

SCH: Understanding how to manage Lily, when to use her Eos over Selene (I really love the silence and SUPER appreciate having it,) and how/when to position her properly is the most rewarding part of SCH for me! I am NOT a fan of leaving her to her own devices since she's a bit trigger happy. SCH also needs to take note of more resources than the other healers and I feel like this is the highest skill cap healer in the game; though not saying it's the hardest by any means. They work wonderfully with WHM and AST but when you overlap with other SCH it can feel clunky without proper communication.

The best thing to do in my opinion is to get 'em both to 70 so you can see the full kit in action! Good news is that the gear overlaps except for weapons so you needn't worry about one falling to far behind the other!

AST is better since we have cards, tell fortunes, and get really amazing pointy hats. That's just science tho.