I’m assuming you are going into A-Levels.
I did both of those subjects in A-Level.
Biology has a lot of content and so you’ll have to do lots of memorizing. There are lots of large 6 mark questions each based on some sort of process.
Biology marks schemes from experience, can be awkward at times - you feel like you’ve got the answer but there’s marks schemes that are very specific.
In AQA Biology - there’s an essay you have to write in paper 3 so consider that too.
Maths (single) in A-Level is generally split into 3 parts. Pure, Statistics and Mechanics. Mechanics is something normally new to GCSE students - it’s similar to physics pretty much. You’ll learn about things like pulleys, objects on slopes, friction, rods and etc. There’s questions normally based around kinematics and suvat, or using newtons laws or calculating moments. In statistics you will normally have to learn multiple hypothesis tests. They are pretty basic normally because they follow the same/similar methods each time but some of the later content in year 2 can be confusing. There’s also a small amount related to the large data set and methods of collecting data but most of that part is just memorizing.
Pure was my favorite part of A-Level Maths - you’ll learn about differentiation, integration, arithmetic/geometric sequences among other sequences, vectors, using binomial expansions to expand things, a variety of trig identities including cot sec and Cosec, there’s modelling questions with all of these topics, there’s also a bit of proof that you need to be comfortable with, there’s also logs and exponentials and coordinate geometry. Don’t get intimidated by all these words - they’ll make sense in time.
Please note that topics can vary between exam boards but those are generally what A-Level maths consist of.
For me - I found Maths to be more fun just because there’s either a right or wrong answer and because it was my favorite at GCSE. In Biology you need to try and pick the right way to say things to get the marks and memorizing can get tiring . Both in my opinion are still very interesting subjects which can open up a multitude of different degree options. You should consider the type of career you want in the future too.
Goodluck