I worked in academic research for a few years after university and gave that up to train as a primary school teacher. Although I would have preferred to teach the middle school age range (8-14 ish) that wasn't an option and I knew I wanted to be a class teacher rather than a (science) subject specialist so I went for primary (KS2).
Before worrying too much about which route to take into teaching I'd definitely try to spend some time in schools, both primary and secondary. National science week is coming up in March so it wouldn't be that difficult to get in as a real-life scientist for children to ask questions of.
If, 12 years ago, you were worried about the behaviour management side of secondary teaching then I'd recommend making sure that the schools you spend time in represent a real mixture e.g. mixed intake, socially deprived area, high proportion of children with EAL rather than a local school full of middle class children as you never know where you'll end up!
Secondary is great if you want to specialise in your subject but you may find it frustrating to have to teach your specialism to repeated groups of students for approx. 5 hours a week (at GCSE level) and will almost certainly have to study some chemistry and physics (I know when I did my biology degree that there were some PGCE students studying various units with us because their degrees were in physics so had to get themselves up to degree level in the other two subjects).
Primary is fantastic from the point of view of being able to form a relationship with your own class and tailor teaching to their needs, interests and personalities. I love that I can change the structure of the day to suit what we're learning e.g. when studying the movement of the sun I can take them outside repeatedly to measure shadows at different times of day whereas secondary teachers are limited to a single (or maybe double) lesson. The cross-curricular nature of primary teaching is a real strength too as subjects don't need to be taught in isolation - if I'm teaching data handling we can use data from a science investigation, combine it with an ICT session to show how Excel can be used and the literacy input can be related to forming a hypothesis, writing methods/conclusions etc.
Day-to-day primary is a lot more demanding as you are with the same group of children all the time (primary teachers get 10% planning time but no free lessons whereas secondary teachers get 10% planning time plus free lessons) but that can be easier than having to learn the names and needs of 300+ students across KS3/4. Primary teachers only have a couple of parents' evenings a year whereas secondary teachers will have a couple a year per year group and the same is true for reports although in secondary you will only be writing about your subject instead of absolutely everything. Primary teachers are expected to take on a curriculum area as a responsibility once they've completed their NQT year and unless this is a core subject there will be no additional pay/time given whereas secondary schools have more staff and are organised in departments so the workload is shared.
Behaviour management is an issue regardless of which age you teach; I encounter far more challenging behaviour in the primary school I teach in (deprived estate in inner London) than my secondary school teaching DH does (grammar school in outer London) but I enjoy teaching in my school and would probably be hopeless in a grammar school! When it comes to choosing a course do make sure that you choose one which will give you proper behaviour management input. Lanky 15 year olds may be scary but so are weapon-wielding 10 year olds so don't be fooled into thinking that it is easier to manage the behaviour of younger children. It will depend upon your own skills, the school's systems and the children.
In terms of courses you will also want to check if all of them mean you are entitled to the golden hello schemes; when I did the GTP I received a training salary (taxable) and was then not eligible for the golden hellos. By the end of the year I was no better/worse off than those who had completed a PGCE but had had the advantage of being paid a monthly salary which helped with the mortgage. Your employability will depend upon the area you live in. There are always jobs in London but the South West, including Bristol seems to be overrun with teachers (and has a fresh supply of cheap ones each year due to the high volume of training institutions they have down there).
Contact some local schools and ask if you could shadow a member of staff for a day, ideally someone who teaches the full range of students and that may help you to make a decision.
Personally I think teaching is the best job in the world; it's hard work, frustrating and can be emotionally draining but is a truly privileged position to be in and is a lot of fun (eventually!).