Both are serious platforms. They're built for different goals.
Brilliant is a well-designed platform with genuinely high production values. Its strength is interactive problem-solving — it teaches through puzzles and guided discovery, and it covers a wide range of topics including math, science, and computer science. If you enjoy being walked through a clever problem and working toward an insight, Brilliant does that very well.
Brilliant is built around intuition-first learning — you explore a concept through a puzzle before it's formally defined. MathJourney is built around foundation-first learning — you understand the concept fully through a written explanation before you practice it. Neither approach is wrong. But they serve different learners. If you want to feel the shape of an idea before naming it, Brilliant. If you want to understand exactly what something is and why before working with it, MathJourney.
Brilliant's content is organised by topic but not as a strict linear progression — you can move around relatively freely. MathJourney's curriculum is a deliberate sequence. Each course builds on the previous one, and each lesson within a course builds on the previous lessons. For learners starting from scratch or rebuilding a shaky foundation, that structure matters. You're never placed somewhere without the preparation to be there.
Brilliant includes practice problems within its modules, but doesn't have a dedicated long-term retention system. MathJourney's spaced repetition engine continues working after you finish a lesson — scheduling reviews weeks and months later to make sure what you learned stays learned. For subjects like mathematics where cumulative knowledge is everything, this is a significant practical difference.
If you want a broad range of topics across math and science with an emphasis on intuitive problem-solving, Brilliant is a strong choice. If you want a structured, deep path through mathematics specifically — from foundations to advanced — with a system that ensures long-term retention, MathJourney is built for exactly that. The two aren't mutually exclusive, but if you can only pick one for serious mathematical progress, that's the distinction.