Both are hot springs near Karakol, and that's about where the similarity ends.
Altyn Arashan sits deep in its own valley, reached only by a rough 4x4 track — the springs are simpler, more rustic, and the appeal is really the whole valley setting rather than the pools themselves. Ak-Suu (also called Teplokluchenka) is far more accessible — no 4x4 needed, more developed pools, and a shorter, easier trip overall.
If you want a scenic mountain valley adventure with hot springs as part of the package, go Altyn Arashan. If you specifically want a relaxed half-day soak without the rough road, Ak-Suu is the better fit — and it pairs easily with another activity the same day, which Altyn Arashan's longer drive doesn't really allow.
Zoomy