First of all, I would definitely tell you, DO NOT PICK YOUR MAJOR BASED ON WHICH ONE IS EASIER. Study something you're more interested in. If you're going to be a freshman this upcoming fall, you may not even be pre-med after this upcoming year; I know you probably get that a lot, but even if you are pre-med, while you're suffering through your pre-med requisites, your major should be something that interests you, so at 4am when you're doing cramming for ochem and get to your other classes, you are studying something you're interested in.
That being said, I'm a neuroscience major. At my university, I think it's a really challenging major. I think people really underestimate how challenging it is and of the friends I had who started out neuroscience, only a fraction are still in the major. I think microbiology is challenging in its own way, but I would choose neuroscience over any of the other biological science majors offered over the majors at my university in a heartbeat. I originally came into my university psychology, but neuroscience had my heart in a second. My freshman year I took some of the most challenging core classes in my major that weed people out and are offered through the medical school; I literally had classes being taught by medical school faculty that had test averages in the 60s, but the subject matter was so interesting that my motivation to learn it made me want to master the material and ultimately allowed me to do well in the classes. I think if you're truly passionate about learning the core of human behavior, this is an incredibly rewarding major.
I love neuroscience because my major has allowed me to be exposed to research opportunities in neurological disease, which fascinates me and was an incredible learning opportunity for me able to figure out whether I wanted to pursue a career in medicine or a graduate program. I also love it because it allowed me to really challenge myself. The amount of knowledge I have gained about the way humans work, about how everything we do is literally controlled by intricacies in an organ the size of the fist - that has inspired my passion for the sciences and inspired me to continue to pursue a career in medicine. So at 4 am, when I am frustrated with my premed requisites that I have to take and I have to study for a graduate seminar in my major, I don't mind it as much, because it reminds me why I'm really putting myself through the other classes - the end goal is much more apparent to me. I love what I study, and I hope that you're able to find that within Microbiology or Neuroscience.
You definitely don't have to figure it out right now, either. If you're going to be a freshman, take an introductory class in perhaps Behavioral Neuroscience and Microbiology along with the premedical coursework you have, and see which one you enjoy more. The journey is half the fun. I think my major has prepared for medical school well because a lot of my pre-medical coursework was a part of my major requirements, but I really wouldn't worry about that in picking a major; in working with an advisor at your university, you can find a way to make your premedical requirements fit what you need to take to be prepared for the MCAT.
Really, just major in something you love. That's my biggest advice to you. Whether that be Microbiology or Neuroscience.
best of luck!