Maths school vs. independent (private) sixth form
Independent (fee-paying) sixth forms and university maths schools sometimes end up on the same shortlist, particularly for families focused on Oxbridge and top-Russell-Group destinations. They're solving different problems, though.
Cost
The nine open university maths schools are state-funded free schools — there are no tuition fees. Independent sixth forms charge fees that can run into the tens of thousands of pounds a year. This is usually the first-order decision for most families before subject range or ethos even come into it.
Breadth vs. depth
Independent schools typically offer a much broader subject range, extensive extracurricular provision, and established boarding infrastructure. Maths schools offer the opposite trade: a narrow, deep, maths-first curriculum with a small, self-selected cohort, but little else in the way of subject breadth or facilities beyond what the sponsoring university and college partner can offer.
Peer group and selection
Both models are selective, but on different criteria: independent schools mostly select on ability to pay plus some level of academic entry requirement (which varies hugely by school); maths schools select specifically on demonstrated mathematical aptitude, regardless of family income.
For a family weighing a fee-paying sixth form purely to access strong maths teaching and an Oxbridge-oriented peer group, a maths school can offer a genuinely comparable — sometimes more intensely specialised — experience at no cost, provided the narrower subject range and (often) longer commute are acceptable trade-offs.
Related: maths school vs. normal sixth form, boarding and travel.