A few comments from me as an ST1 tutor (for what it’s worth!).
Definitely agree with Zebedee, you must take the combination of exams that most interests you. Some students will have an eye on their future career plans too (but I’d like to think that in most cases this would be consistent with one’s interests

). The examiners try to ensure that the ST subjects are of broadly equal difficulty in any case. A lot of students tell me they find ST1 more interesting that ST2, once they get into it.
There is certainly some overlap in the core reading between ST1 and ST2. However, there are important differences and additions. I see all too many students who take ST1, thinking it will be “easy” because they have already studies ST2, then come a cropper because they haven’t done enough preparation.
Apart from the occasional bookwork question, ST1 exam questions generally have different emphasis from ST2 – e.g. they may expect you to include more on product specifics or meeting customer needs. This is not difficult so long as you’re aware that it’s not the same as ST2 and you know your stuff.
Note that the chapter on reinsurance is, in some respects, different from ST2 (at least what I remember of ST2). For example, it deals with surplus reinsurance differently (there’s no reducing retention), and it includes more on non-proportional reinsurance (e.g. XL) and financial reinsurance (for short-term insurers).
Hope this is of some help - the choice is up to you!