If you are a Phys NatSci there is almost no point in reading this, as the choice that awaits you is doubtless “A or B?”. My advice is simple: start off with the tougher B. If you can cope with the course over the year then it will prepare you much better for the end-of-year exam (which is identical for both courses). The reason is you will cover some theoretically harder material in the B course, but material that is harder in theory has easier exam questions!
If you are struggling with Course B, then switch to Course A, nobody will think any the less of you and it will mean you have much more time for your other subjects. I found Maths took up most of my time so this was maybe something I should have seriously considered!