This may sound a bit twee but the best route is the one that you are happiest with and which you will not resent doing.
That said, there are far more people who did a degree then PGCE (this is the more standard route to QTS) for secondary teaching than do an undergrad degree with QTS, though this is the more common route for primary teaching. For gaining QTS either route would do as QTS is a set of professional standards that at the same for everyone regardless of how they train to teach, undergrad, post grad or GTP.
Most PGCE programmes are now assessed at M (masters) level and will provide you with M level credits which could be used towards gaining a full masters degree.
The PGCE is the most commonly recognised training route for secondary but there can be advantages in the undergraduate route - e.g. placements over a two year period which give more time for consolidation of your teaching and learning.
Subject knowledge is important, but even a BSc in biochem will not deliver the breadth of knowldege and understanding that a science teacher needs, so whichever route you choose there will be a need to widen your science knowledge and understanding. Ultimately it is about what YOU feel is best for you - once you have QTS and gain a post (as a chemistry specialist it would be easier than as a biology specialist due to the shortage of chemistry teachers) then your actual degree qualification fades somewhat.