One thing you should really consider is the workload you have with other subjects as well.
I’m not sure why other subjects you’re doing, but my massive assumption from your name tells me you’d be doing some very heavy subjects on the side.
It is really nice to have a subject you can put relatively less effort into and still get a decent score in. It allows for more focus on other subjects while maintaining score.
For me this was both Further Maths and Legal studies. Collectively I’d say I put about 70% of my effort into the rest of my subjects, 20% into further and 10% into legal studies. It really went a long way in allowing me time to put into the others.
Lastly, take this with a pinch of salt due to its anecdotal nature, but I went to a selective school like yourself and from what I’ve heard and been told, a general rule is that it’s easier to get a 45 in Further Mathematics than a 30 in specialist. I’m not sure how reliable that is ¯\_(ツ)_/ but it seems to me at least that unless you’re very interested in specialist, further might be the way to go.
Also, I might be a bit biased.