Japanese GCSE or JLPT ?

Japanese GCSE is more straightforward. You do the reading, speaking etc. on the set days and then eagerly wait for your results. JLPT takes more time since you have to decide what level you’re at, it generally expects more from you and honestly a qualification below N2 isn’t worth the money. It depends what you want the qualification for. If you’re planning on studying/working in Japan, spending a little longer for the JLPT is more rewarding. If it’s to put on your university application in the UK, go for the GCSE.

But, you can always do both?

I just found out I'm going to be able to do Japanese (s/hl) at IB next year so I won't be needing either, but thanks so much for the advice!

Japanese at GCSE and A-level is ridiculous if you want to actually work with the language, you'll need to pass N2 at least eventually. There is, however, an argument for taking A-level Japanese and that's for entry into a Japanese course at university, if not then bear in mind that N4 is like A-level and N5 is GCSE and in my experience the progression through the levels is not linear so you can except N3 and N2 to take way longer