I don't know which one is the hardest but Edexcel is definitely harder than AQA. In AQA you have so much content that is similar to content covered in the M1 and M2 Maths modules.
AQA B because it's full of irrelevant stuff and there's hardly much revision material for it apart from its textbook from the examiners of this board but there are many errors in it. It's not much maths-y and for me I found this a disadvantage because explaining things is my weakest point and I generally liked doing calculations more. In others words AQA B is wiffly waffly crap - avoid it at all costs.
I've had to change sixth form because of this exam board and now I am restarting again. I am now doing GCE Physics WJEC which is much nicer imo