Advantage of pure maths courses in transcript for physics grad school
I think the real answer is that it depends on what it is that you're planning to do. The more closely your courses align with your proposed field of study, the easier it is for the committee to evaluate. So, taking pure mathematics courses makes more sense if you're going into a theory-based field than if you're doing something more experimental in focus. 

However, you should also keep in mind that a strong grade in an upper-level mathematics course will at best be "neutral," and it might be possible to explain away a bad grade; a bad grade in any physics course will not go over so well. So you should factor your interest level and ability to handle the courses in your selection process. You should also consider if the course in physics is considered a "standard" course for undergraduates pursuing graduate admissions. If so, then it makes more sense to take that class than an elective mathematics course. (Otherwise, the question will of course be "Why didn't you take course X?")