Probably immuno, but really neither is necessary. Take what you're interested in. You'll learn everything you need to know when you actually get to med school.
Seconding @WedgeDawg , but I would add that though you'll learn this stuff in med school your life might be easier if you already knew it. In light of that, I'd go with Immunobiology since its a more difficult/intricate and high-yield subject than pathogenisis.
Immuno. Wasn't taught well at my school and I had to learn it myself. Going in depth into immunology during undergrad is very smart as it seems that many diseases have their basis in immunology, genetics and infectious disease. Relevant for every field but particularly allergy, rheumatology, pediatric nephrology, hematology/oncology
Immunobiology. Of the courses I took in undergrad, I believe the following have helped or will help me the most in med school: medical microbiology, immunology, pharmacology, endocrinology, and molecular basis of human disease.