I don't know whether any are more difficult but they are different. DS1 is doing AQA Maths and FM and it seems an odd method. They do the two side by side and do A level Maths in year 12 and A level FM in year 13.
Yes I didn't explain it very well, it's the same I think. He does a minimum of 13 modules and at the end of year 12 they have technically got all the A level Maths done although I don't think they bank them. They can demonstrate an actual A level grade on the UCAS application though. He wants to do Maths degree.
There are two types of OCR maths A level - MEI and the non-MEI one. I have heard it said that MEI is quite a hard option, also that Edexcel is too, but all anecdotal and not especially reliable sources.
The non-MEI OCR syllabus is one of the most popular ones, so presumably schools choose it either because they think it is easier, or else easier/nicer to teach. The MEI syllabus is less similar to the others in that it still has some coursework (though that may be optional) - whether or not that makes it harder is subjective.
Generally many dc tend to think their syllabus is the hardest, I suspect there's not that much to choose between any of them. Most (if not all) can be done either in the way secret describes (with the maths and f. maths sequentially) or with the maths and f. maths being done in tandem (AS for each the first year, with the A2s the second). Being able to 'juggle' a number of the modules between the different qualifications to optimise the results is the way all of them work too.