If I were your advisor, I would say take P-chem. You are going to learn a great deal of fundamental concepts in physical chemistry. The biophysical chemistry class is not going to be as rigorous and will be more of an applied class. You are not going to learn the fundamentals in very much detail. But you will be able to apply the things you learn in p-chem to biochemical processes, especially when it comes to thermodynamics.
I haven't taken biophysical chemistry (which is only offered as a graduate level course here) so I am not sure which one is going to be more difficult. But, if I had a to guess, the p-chem class would be more rigorous and probably more difficult. But, it will look better on your transcripts and when you apply for jobs or grad. school, it will work in your favor. If you do go to grad. school or if you have room in your schedule later, you can always take the biophysical course. I'm sure it would be beneficial for you given your area of concentration. But the p-chem class is more fundamental and I would advise against substituting it.
Hope that helps some