Eh. I would probably take Immuno. For the most part, the big hump in Immuno is the terminology and getting the concepts. For me, when I learn concepts it tends to stick better/longer and it's applicable to other things as well. However, immuno, at least at my school, is a small part of the overall curriculum.
On the other hand, in my experience Micro is mostly memorization. If you are good at that and think you'll remember some of it when you get to med school, then go for it. My only concern is that you won't really know the focus of the undergrad micro class until you are in it. I.e. are they going to focus on the major microorganisms that you would in med school? How much detail are they going to go into regarding each microbe? etc. etc.Micro, at my school, is a slightly larger part of the curriculum. However, unless you can accurately gauge what you'll cover in the class, it's hard to say in the grand scheme of things if the UG class would be of any use in the future.
There's always the fall back of taking the one that you'll find more interesting. That's usually a win-win.
