Hello - I have experience working with both age ranges. I have more full-time experience in primary but have had part-time secondary experience in leadership and pastoral roles so hopefully this does not appear biased.
It's good that you are thinking about your choice critically and weighing up both options. They are definitely different jobs and use a very different approach to teaching and learning. In some ways, however, the jobs are much the same. I'll try my best to highlight - what I think - are the most important differences between the two.
1.
Age is obviously the key difference. If you are taking secondary then expect to work with 17 and 18 year olds, whether you are looking to teach KS5 or not you will still be working in their school and you will be a subject expert, they can and likely will come to you for advice etc. Similarly, if you are teaching primary expect to work with children who are just embarking on their journey in education (those who can't write their name or wipe their bottom for that matter!) - joking aside, reception year is a pleasure to teach.
2.
Subject. In secondary you will teach your subject to a range of ages - which has its difficulties, you will work on getting your pitch right to the different year groups and find your way around exam specifications and will regularly contribute to the evolving curriculum of your school. However, in primary you will be expected to teach the full National Curriculum so be prepared to get your musical instruments, paint brushes, French and your axle and wheel at the ready. This is what I love the most about primary school - it is a hub of diversity and on Monday I'm a scientist but the next day I might be an artist.
3.
Work. The work is different in some ways. In primary, you will have your own class and those children stay with you everyday - all day. This means you need to ensure that you have things planned to do for the day. There is no changing of period 1 to period 2 where you have to fill one hour with Year 8. You must be more flexible. Also, you are responsible for all of those children in a pastoral way too. If you are experiencing safeguarding issues or mental health problems then it's likely to fall on your shoulders. In secondary, you teach many more children the same subject. However, your school will usually have a pastoral department or student support that picks up those who are having particular challenges. You may, in secondary, have to deal with more exams etc. so that work balances out. In secondary, the planning is different. You might plan a lesson for year 8, but you teach four year 8 classes so that's four in one. However, in primary that doesn't quite work the same as you only have one class. Likewise, older children are more unpredictable with how much you will get through in one hour so it may take more of your time planning extension tasks etc. A recent study from UCL involving 40,000 teachers found the working hours of primary teachers just slightly higher than secondary - you can read the full study here:
https://www.bbc.co.uk/news/education-49728831 4.
Size. The size is the other important factor to consider. Most secondary schools are large in both the pupils on roll and their staff community. As a member of staff, you are in a very big pond indeed. Of course, this differs from school to school and it is important to find the school for you. Many primary schools - excluding London - are between 1 and 3 forms of entry and this means they're smaller in comparison to secondary. You will be in a much smaller pond of staff. This has its pros and cons and is really down to your personal preference.
Whether you decide on primary or secondary it is the best job in the world, enjoy it.
Hope that helps. Sorry for the long post, I am going crazy as schools are closed!