On balance, is it more valuable to review for a journal you haven't yet reviewed a paper for? I'm a grad student, and over the weekend, received two review requests from very good journals in my field, one of which I have already reviewed for. 

I declined one because it was out of my area, but it made me wonder if, all else being equal I should prioritize reviewing for a journal I haven't yet, or if it doesn't matter.

I have often seen CVs in my field list journals reviewed for, as well as a number of reviews, so it is information that is out there and valued
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.)