To maximize job prospects I would say organic chem would give you a better chance. Often instrumentation PhDs like spectroscopy may get you jobs as an instrument expert or owner which might not be as enjoyable. However, you could also develop ideas for improvements in the instrumentation technology and help new technology companies release new products or start your own start-up.
If you are in the UK, which sounds likely given your grading expectation, I would recommend going straight for PhD programs rather than doing a master's first. In general, I always advise people to finish as fast as they can and get out of academia, where they will be more highly valued, although in the UK the pay is low in industry compared to much of Europe, and definitely the states. A quick Google is putting the average salary of post docs in the UK is 39k GBP compared to the expected position you would get straight out of a PhD, which is Senior Scientist showing an average of 44k. So likely you would start at lower than that for either positions.
Contrast that to the states where the average post doc is getting 60000USD (49.5k GBP) and average senior scientist 101000USD (91k GBP). Best prospects for money would be a move to the States, but only if you want to go there and many companies are in very expensive areas, Boston, New York, California. The salary is great and you will save a lot if you are a single person, I myself earned a lot of money in Vancouver before reaching breaking point with three kids, unable to afford a 3 bedroom house, moved to Sweden where we just bought a 4 bedroom detached house with a huge garden next to a lake for the same price we sold out 2 bed townhouse that was so narrow we called it a corridor.