From personal experience (ex Primary, now in different sector) DH who was ex primary now FE and many many friends and relatives in all sectors, if she has the skills and qualifications, then do secondary maths. There is decent financial support and more jobs. There are swings and roundabouts with primary/secondary but I'd say primary requires more hours on the school premises as you also have an environment to keep up to high standards as well as everything else - while teachers aren't supposed to be putting up displays, the reality for many is that their TA hours are so committed to group and individual work that it's the only way they get done. To create a high quality (regularly changing- some schools demand every half term or term - that could be 24 interactive displays in one school year) learning environment requires a lot of work and imagination actually physically in the classroom (usually halfway up a ladder.)
There also tends to be more differentiation in the average primary class than secondary, where groups and sets reduce the massive differences in ability and attainment (especially in maths, not so much in e.g. RE, ICT) which means you can spend your whole life trying to personalise the learning for the children in your class.
Anecdotally, PPA is more useful to secondary teachers as well. Certainly many teachers in primaries, because they are responsible for all the learning in that class, have to set and mark the work done during PPA, which is often covered by a non-teacher for budget reasons. I know this isn't supposed to happen, but it does.
There also tends to be a more defined management structure in secondaries with different people having responsibility for subject lead, or pastoral. In smaller primaries it's not unusual for a class teacher to also be leading 2 or 3 subjects for the whole school, with neither pay nor time allocated to carry this out.
There is active recruitment for secondary STEM teachers at the moment. For someone who is genuinely interested, I'd say go for secondary. My secondary friends still work all the hours God sends, but manage to fit it round their families more effectively than many of the primary teachers I know.