All the boards are 50% Further Core and 50%Applied, I haven't checked that the FCore is the same but I'm assuming it is (usual AssUME rules apply).
OCR A and AQA require that you do two of Decision, Stats or Mechanics.
Edexcel allows you to double up, so you could do D1+D2 or M1+M2 or S1+S2 - however doing M2 or S2 would be significantly harder than doing D1. I could see that a Physicist would rather do M2 or a Computer Scientist may prefer D2.
OCR B (MEI) makes you choose between one Major option (Stats or Mechanics) and one of a list of minor options. You Can choose three minor options, so Decision*, Numerical Methods** and Minor Mechanics (or Stats) would be an option and would probably be an "easier" option (as Numerical Methods used to be an AS option as was Decision*). They also offer Extra Pure and Further Pure with Technology. That said you would need to check that the Core is the same and compare how much is covered in the Minor option vs FM1 for Edexcel.
With this in mind, it also depends on the type of mathematician you are - I have met people who found D1 tricky because of time management to get through the whole paper.
* They don't call the Minor option "Decision" they call it something like Modelling and Algorithms.
** Numerical Methods will also cover first principles Differentiation and trapesium rule integration, which I think you cover in normal Maths at Edexcel.