I think that the only reason you should be taking courses in mathematics is for your interest. Put differently, you should not take these courses in order to make your grad application look stronger. If you want to learn more about statistics and probability, but you are unsure of your ability, take the first. If you are more ambitious and willing to put in work for the sole sake of knowledge, take the more advanced one.
Again, I don't regret venturing into the Mathematics department. But I did so knowing that I was doing it for its intrinsic, not instrumental, value.