Hey,
If I was you and you’re target grade is either a 4/5 then I would recommend doing the foundation. However, if your target grade is a 6 and above but you are achieving a grade 4/5 then I would also recommend to the foundation. Although looking at the grade boundaries makes higher look easier to get a 5, it’s actually not because the subject content is much more difficult whereas on foundation, some formulas will already be provided for you depending on what the question is testing and it’s what’s on the higher but in a lot more basic standard.
Some people at my school who do higher got asked if they want to try the foundation tried it and have now said that they are glad they chose to do the foundation.
Foundation is also a lot less pressure.
I hope this helps, I’m happy to explain further if you wish :-).
Pat :-)