Well, the good news is that both are in high demand.
The last few schools I've worked at are persistently short of Maths teachers, often also of Science teachers (though my current department actually has a surplus of Physics teachers and is short of Chemistry instead).
Getting to 50k will be the harder part. Whether or not you will get annual increments will depend on the school you work in, how often you change schools and whether there is any money. Some schools use the 2-year ECT to pay new teachers at MPS1 for the whole time, for example, others will make appraisals with such unrealistic targets that few if any achieve them. Changing schools can mean staying on the same pay rung for a while, or even dropping, depending on what is offered.
You don't need to be a Head of Department to get a decent TLR (I'm not and have successfully circumvented the role for many years now despite a few offers). To be honest, it's a job I really wouldn't want, especially in Science.
While Maths is seen as the more important Core subject, and therefore has more pressure, it also has far more support, both from SLT (e.g. first dibs at student lists, priority at interventions), and from students and parents with their efforts to succeed.
The sciences are lumped together despite needing different skills, you have a large team plus technicians to support, the kids often don't care because they "don't need it to get into college" and despite being a core subject, you will have little say over decisions made by Maths and English when it comes to set lists, intervention slots etc.