My DS is at an IB-only school, but sorry quite a way off from 6th form.
One thing I can say though is that years and years ago, when I did A'level - just 3 - I had far two much spare time: I could have managed a 4th in the second year plus more sports/activities etc. Some schools may be better at a fuller, rounder programme nowadays, or may be not, but IB does put a structure around all that wih formal recognition of it too.
IB seems more likely to ensure better value out of the education system than A'level. Subjects like music and art probably dominate any schedule though and possibly preclude some other choices for most, simply by the amount of creative coursework they demand by the time required for them. For example, it migt be a rare child doing triple science, music and art, and besides the school's funding might restrict options.
Just thoughts mind you and part-digested regurgitations from others' past posts!