No, biology is definitely not a subjective subject, it is completely unbiased and relies on straightforward answers. However, it also in a way depends on who marks your paper, there are always going to be variations, but this is not a problem as long as you ALWAYS include key terms and show them that you know what you're talking about. It is a difficult subject because you always have elaborate and provide specifics (e.g. key words), else you either lose marks or just simply not gain any marks at all. In fact the marking scheme is actually kind of annoying because most marks are lost not because people answered incorrectly, but because they did not provide what the examiner was looking for i.e. elaborated responses and KEY WORDS!
I personally think it is better to do 3/4 biology in year 11 since it has a pretty huge workload which consists of memorisation AND understanding the concepts. This means that you have more time to study for it etc.. However, as far as I'm concerned, methods also has a large workload and is difficult, and regardless of the study score, doing either early is beneficial since they both reduce the workload heavily in year 12.
Judging by your question, I'm assuming that you're in year 11 right now doing unit 1&2 subjects (sorry if I'm wrong :S). But next years midyear exams will be axed and I think you should ask people who've completed 3/4 methods how well the two units relate to each other. I don't know about methods, but in biology, unit 3 and unit 4 have very little in common and are only linked by a few topics, so I would definitely keep that in mind.