Neither is preferred, so do whichever you think you'll do better in. Anecdotally I've heard A-levels are a bit easier to get top grades in than IB (as I knew some people who did IB then an A-level or two in a gap year) but in principle they're both equally acceptable and if you are a highly achieving student you'll probably do well in either. That said anecdotally in my experience and from everyone I've known doing IB, it is a higher ongoing workload than A-levels generally as you just have all your timetable filled in with lessons for the most part. So it's certainly intensive as courses go!
Although I did hear, in terms of course preparation, IB was useful for a medic I indirectly knew as she had to do essay writing during IB and compared to all the other students on her medicine course who did all STEM subjects at A-level, she was much better at writing essays for her course (and had to teach her friends how to properly structure an essay!). This was an n=1 case of course, and might be specific to the medical school she was at (Cambridge, which involves more essay writing due to the tutorials I gather) but was something that might interesting to consider.