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.

>>3
An undergrad physics degree is your best preparation for grad school.  None of the upper level math classes I've taken have been crucial.  You've taken diff eqs (I assume with proofs and stuff), so you're probably already ahead of me.

real analysis - I understand limits/calculus better, but useless
complex analysis - supposedly useful in QFT, but I haven't had the need to do contour integrals yet
numerical analysis - vaguely enriching, but useless
linear algebra - how did I pass this class?
abstract algebra - some use in QM/QFT, but physicists are interested in representation theory, which mathematicians completely ignore