I don't know much about the IB (I didn't even know there weren't exams at the end of Year 12) but friends who did it have told me it's more work than A-levels because you have to do extra stuff like community service. It really depends whether you want to take on an extra subject but study all 6 in less depth and 3 at a lower level, or stick with 5 and go into more depth. In terms of university and your future career, it doesn't matter as long as you take the subject you want to teach (or I think any National Curricululm subject for primary) at higher level because unis accept both. You could do some research into what exactly you'd be studying for each qualification because the topics might be different at A-level compared to IB, and obviously the assessment is different as well. Just out of curiosity, why did you choose French and economics for IB, but politics for AS?