I started this course with initially no intention of doing Geology, and almost no prior knowledge of the subject, but I enjoyed the course immensely, and nearly continued Geology into Part II. The course covers a wide range of topics, from petrology to paleontology, which means that there might be parts of the course which you find tedious, but I found even the topics I expected to be dull quite interesting.
The more bio-oriented sections I found fairly straightforward despite doing Chemistry and Materials as my other options. Many people worry about the essay based nature of the course, but there are analytical questions on the exams, and the essays are more about explaining the concepts in a concise manner rather than constructing a literary masterpiece.
Fieldwork (a week or so on the Isle of Arran) is a required part of the course, but most of the people I know, including myself, found it one of the most enjoyable aspects of the subject, and it’s a really good way to make mates outside of college.
I’d recommend this course as a third option of you’re not sure what to do, as it is really enjoyable, even if you never look at another rock again in your life.