When I read student evaluations of someone's teaching, I look at a few different things. First, are there concrete examples of problems that show up consistently in all the evaluations, or does every student just express their own pet peeves? Second, does the student writing the evaluation sound like they actually put in effort in the class, or are they just complaining because it wasn't an easy A? Third, do any strange things jump out that are atypical to see in a teaching evaluation that indicate a serious problem.
In these examples, I see a mixture of those. The majority sound like typical below average students whining because they are expected to know more material than just what is spoon-fed to them in lectures. That is typical for college level biology courses to need to learn a lot of the details on your own by reading the textbook, and only highlights and broad concepts are covered in class. Those complaints are also coming from the students who can't write a single sentence without egregious spelling and grammar errors. They're the whiners who are going to gripe on every medium they can find.
A positive thing I see in the evaluations is that it has come across to at least some of the students that this professor does love his subject and that if you are confused about something, you can just ask, which means he's approachable. The downside is it sounds like a lot of students are confused...there are consistent comments that he strays from the lecture outline frequently. This certainly can lead to confusion, although if you're learning mostly online, there will naturally be more structure from that format. If you find he does stray from the outline often, as long as you're an attentive student and good at taking notes, that really shouldn't present too much of a problem. It might require staying after class once in a while to get clarification of how topics are related if he went off on tangents that distracted from making those connections. It sounds like it would also be wise to ask if material presented during those tangential discussions is part of the required learning for the class, or just supplemental material for your own edification. Likewise, I'd directly ask if the material that he does not get around to covering is still something he expects you to learn on your own for the tests.
The only strange thing I'm seeing in those evaluations are the comments from those who seem to have gotten A's in the class who are saying you can think you've failed every test, but if you see him in office hours and look like you're putting in effort, you'll still get an A. This can mean one of two things. Either, he really wants to push the students, so grades harshly to get them to work harder, but in the end is an old softy who bumps all those grades up so he's not really failing half the class. Given the remarks that he should retire, I'm assuming he's an older professor, so this type of attitude is not uncommon...sadly, I hear from many older faculty members that they actually used to get many more students who DID work much harder and were much more motivated by those approaches than current students are. The alternative is that there is something horribly wrong with the way he grades...either giving favoritism to students who show up at his office, or the feedback they get on their exams just doesn't give them a clear indication of where they stand. Of course, it's also possible that he just gets annoyed with the students who are perpetually asking about their grades rather than learning for the sake of learning, so tells them all they're failing to get them to stop focusing on grades instead of learning.
On the other hand, if the other class does not have someone assigned to teach it yet, this late in the game, you're likely to get someone who was just stuck with the assignment last minute who may have never taught the course before or who is unhappy about having to teach (sometimes those last minute assignments go to the people who are not teaching faculty, but are the research faculty in a slump on their grants, so the department assigns them to fill in any teaching gaps to make up for the funding they aren't bringing in at the time). Or, you might get the same person teaching both classes. You just never know.
Knowing your work ethic and how well you learned the material in your last bio class, I'd lean toward saying you're probably going to be better off with the class that better fits your schedule and taking your chances on the older prof who enjoys his subject. Forewarned is forearmed...if he starts to go off on crazy tangents during the lecture, you'll know to expect it. You'll also know that he is not going to just test on what is presented in lecture, but material likely found in textbooks or other supplemental reading material. If you're conscientious and study the textbook plus pay careful attention in lectures and find that material is still showing up on the exam that has nothing to do with anything covered for any part of the course requirements, I'd suggest planning to meet with him right after the first exam and ask where that material came from. This should either give you insight into where he's drawing those additional questions from (I used to hear this complaint all the time from students who read all the text in the textbook, but never bothered to look at the figures, so couldn't answer the test questions based on information provided in the figure legend), or alert him that there are questions on the exam that perhaps should be excluded from the grading.
