Mathematics for physicists

I was thinking of double majoring with mathematics to complement my bachelor's in physics, but this would end up extending my graduation by a year (two semesters), cost more tuition over my financial aid, and make me double up/ add another class onto my normal fall/spring credits. Would I just be better off studying more mathematics on my own time during the summer to be better prepared for graduate school? If anything I'd love to study both in graduate school but I don't really think I can juggle both at this point of academia. Again, just for my bachelor's.

>>2
Either. I see myself in labs doing research and such, but if I can balance both in graduate school, probably theoretical. My university's highest required math for Physics is differential equations. I guess I can do a math minor and only have to take linear algebra and a higher upper level one.

I plan to just get names of the textbooks from the mathematics catalog and during the summer just go over some of it myself if I don't get REUs. I just want to be best prepared for graduate school.