My 2 sons did IB, their school did not offer anything else. My nieces and friends' children do/did A levels. Honestly, if we could choose again I would probably go for A levels.
IB seems to have a much greater workload and requires a great ability to be organised. Certainly in schools which offer both, the contact hours for IB are longer and the homework possibly more (a bit subject dependent). The depth that A levels offer is very similar to the standard of the Higher Level subjects (again subject dependent, some IB HLs are reckoned to be more challenging) so the often used explanation that IB offers more breadth while A levels more depth is arguable.
There is coursework that takes a huge amount of time but most of the marks (most subjects) come from the exams. These take place in one block over 2 and half weeks at the end. There is nothing stopping an exam schedule (it varies each year) being morning and afternoon for a week solid. You don't want to be below par that week! The way exam questions are set means they really do need to have all 2 years work in their heads right then.
There are very many ways to fail an IB Diploma. It is a scary reality that the whole thing is endangered by one slip.
The other biggie which makes me lean towards A levels is university admissions in the UK. Most of the better regarded ones ignore the tariff as suggested by UCAS and do imo give more challenging conditions for IB takers. There are exceptions so doing some research regarding specific subjects at specific universities might throw up a pleasant surprise. Anecdotal evidence suggests a near-miss at IB is more likely to get you accepted onto the course anyway if the university has any 'spare' places (a dropped IB point looking like a smaller miss than a whole A level grade).
On the other hand IB is a really great programme imo. It gave my kids loads of confidence and I'd say it did make them well set to start uni because they were used to organising their study and well used to giving presentations etc (poss more so than some of their A level peers). However this difference would be negligible by second year and universities are geared to supporting A level students through this anyway.
Having typed that all out (and thought of a few other things)I've convinced myself I'd choose A levels for myself! Er, which I actually did do way back when.