I don't think there's an easy answer to this.
Bright kids who have a wide range of subject interests - France, hands down. My DD could never have chosen 3 A levels. She's in her first year of higher ed (in France) and is only now starting to narrow down her options - and it is likely that she won't be taking the specialisation she had envisaged (she was going to do social sciences but has discovered a passion for history and history of art!)
The new bac has changed things a bit, but it's still broader than in the UK and there are good higher education options that enable bright students to delay specialisation. My DS isn't sure yet if he wants to do maths, physics, engineering or computer science and he will have at least 2 years post-bac to gradually whittle down his options.
For kids with any learning disabilities then I would have said the UK is better, although this may be changing (increasingly under-resourced SEN provision in the UK and some modest improvements in attitudes here).
A lot depends on your local schools too. My kids went to a small public collège (middle school) with mostly great teachers but a very deprived intake. Overall very good school with a high "added value". Other collèges local to us vary from OK to complete jungles.
Lycée has been all right, it's a very mediocre school in terms of results, but the teaching is mostly competent and my DD got her bac with an exceptionally high grade. However my kids are academically able and navigating the French system is a lot easier with kids who get good results without too much effort. We have shamelessly used options to improve the chances of our kids being in half-decent classes - Latin and option bilangue at collège, bachibac (double French-Spanish diploma) for DD at lycée, and an LV3 for DS.