I would say immunology.
Only because in medical school you spend a great deal of time learning about Neurology, but not as much dealing with immunology.
And immunology can be as in depth as neuro.
If you take immuno now, when you fly over it in medical school, you shouldn't have much trouble.
I took immuno in undergrad, myself. For me, it's a waste of time studying immuno now because I covered it in so much more depth last year in college.
Again...neuro will be there in medical school and you will spend weeks on it.
Plus...I think immuno is more interesting anyway.
Just my opinion.