In cases like this, I explicitly preface it with "nit: <text here>", explain how I would do it myself instead and why, and then (also explicitly) say that this is more of a preference thing and that they should feel free to leave it as is. It might turn out they like my approach more and change the code, but if not, it is still totally cool with me, as they learned of just one more way to do the same thing that might come useful later.