Brilliant doesn't have spaced repetition, it has much less coverage and doesn't have enough exercises to let you really master subject, but it's an easy to use mobile app with nice visualizations that can give a conceptual understanding of many things. If MathAcademy and Brilliant covers the same topic, Brilliant may be more visually appealing and contain animations. Brilliant is nice for getting overview of many things, but doesn't really go that deep.

MathAcademy is great for learning new topics that you haven't studied before, provided they have a course in that subject.