We were in the position last year of having to decide whether to do Higher or Foundation for Maths.
DD was working at an 8 in class but getting 3s in mocks. Brought in tutor, maths teacher fully involved and very helpful. Got to the point where she was getting high 5s on Foundation and a mix of 5s and 6s on the Higher.
The final decision was to put her in for the Higher and hope she pulled off the 6 or even a 7.
The Higher papers were horrible. The Foundation papers were nice.
DD ended up with a solid 4.
Her entire class were oddballs who should have been getting higher grades than they were, and teacher trying to suss the issue. They were fairly equally divided between Higher and Foundation on the day.
The kids sitting Foundation in that class all came out with 5s, the kids sitting Higher mainly came out with 4s or 5s.
If I could turn back the clock, she'd be sitting the Foundation papers. Certificates don't indicate which paper was sat.
ETA: At DD's school the decision was made with considerable input from teacher, student and parents as to the best paper to sit. They ran multiple extra mock exams with both tiers of papers before making final choice.