Lamest way to choose a major; you're living the pre-med stereotype bro.
Ease up. He said he was a year behind and that's why he was asking.
Anyways, OP, like the first guy said it's going to depend on so many different factors.
Before you consider biology though, since you're already a year behind have you considered majoring in something else like anthropology, political science, economics, art, a language, literature/history of a time period, journalism/media, sociology, philosophy, psychology etc. You can just choose that and then just do the bare minimum with science classes (bio, chem, physics, orgo, math (some), biochem (some), writing (some). That would probably save you the most time in your squeeze right now. If now then the multiple different factors for difficulty are:
1. Your individual professor. Even at the college I go to currently, there are two sections of Microbiology of varying difficulty. One taught in the fall, for example, is said to be a lot of memorization but straightforward while the one taught it the spring is apparently incredibly difficult w/ the professor asking obscure questions he only covered orally in lecture.
2. Your own preferences. Do you like macro-physiology or do you like cell biochemistry? Personally I'm the guy that loved organic chemistry because it's very logical with common themes and thus like understanding things at the molecular level (i.e Biochemistry), however,
basic nervous system, renal, cardiac, and gi physiology are fascinating as well. Then, for some reason, genetics was a really sore point with genetics for whatever reason (kind of sucks because it's the future of medicine). It wasn't the professor because I'm always in the top 10% of the class but in this class I was I was just a little above average. These are my personal preferences, I don't know what yours could be. Figure them out.
3. The university you attend. More prestigious universities=higher qualifications to get in=chances are students with higher qualifications worked harder and will continue to do so=more competition therefore if students graded on curve, it will be harder to obtain a 4.0. (everyone knows this)
Insert random tangent
[***Interesting note about education: I don't think education is for the sole purpose of getting a job. I hate how politicians among others emphasize education because it will spur innovation and job growth. I like education because it makes me feel better and ultimately makes life a lot more interesting to live. I think school education is commercialized in a way that professors are sadly lowering standards in order to maximize class enrollment therefore $$$ to universities. If I was running political office, I would bring this up, however, no one would care so no change would happen.]
Ok, with those qualifications, based on how it is in my school Microbiology seems more like the memorization of a vast amount of information and same with Developmental Biology but I feel that I would find DB much more interesting because it's like the basis of how our body is. I mean, all our anatomy and physiology is shaped based on what cellular interactions/immigrations took place at the developmental stage. I'd be much more interested to learn about this then microbiology. I think the pre-med should always take the big picture conceptual class over the hardcore memorization class so therefore I'd choose DB because it seems like less memorization and more like something with a logic to it. Things that I'd definitely avoid in undergrad or atleast save until senior year are anatomy and pharmacology. These are both very interesting classes but you send the vast majority of the class memorizing things like the names of each carpal bone or the various types of alpha and beta agonists. I'd much rather take a biochemistry course that I could pick up some logic from to apply to other classes.