Took the MCAT in July ended up with a 521 (130/132/128/131). As you can see Biology was my lowest score. If I could do anything different, I would have taken a physiology course as the material covered in Bio 1 and 2 simply does not suffice.
You don't have to take my word for it. Open up any review book from Berkeley Review, Kaplan, Princeton, or Examkrackers and you will see how heavily emphasized physiology is.
This is strictly from the content list published by AAMC:
As long as you know the fundamental characteristics and particularly the differences between viruses and bacteria you should be okay. The immunology material you need to know you will most likely be covered in more than enough depth during your physiology class. I honestly believe that a semester of physiology will save you quite a bit of review time and of the options you listed your best bet. I know you didn't list it, but if you have not taken Cell Biology, it is a better option than any of those you have listed. People like to say the MCAT is a critical thinking exam, and they're not wrong. The only problem is that you have to have reasonable content base and a good chunk of the passages assume you have decent exposure to physiology.
Don't just take people's advice like this. I highly suggest you open up any MCAT Biology review book and see how many chapters are dedicated to what. That should make your decision pretty clear.