A maths teacher course + QTS = everything you need to become a maths teacher. You will learn what you have to teach and how to teach it. The Qualified Teacher Status will allow you to start applying for jobs straight after your degree (so 3years at uni)
On the other hand: A maths degree will be more maths and less about teaching it however the PGCE (which is an extra year) will charge you and will be done after you graduate in maths. They are highly competitive. The good thing about this option is if you ever want to stop teaching you have your Maths degree to fall back on.
Hope this helps! Good luck x