On balance, is it more valuable to review for a journal you haven't yet reviewed a paper for?
Multiple reviews for a journal indicates that you are a trustworthy
reviewer.
Reviews for various journals indicates that you are known across your
discipline.


Thus, both seem to be positive signals (for different traits). No need to prioritize one aspect over the other.

(By the way, you can have such peer review activities verified at Publons, which may be better than just listing them on your CV.)