Hey, I know I'm a randomer to the thread but I always nose through threads to do with places local to me and I just saw this, haha!
But hmmm, I think you really need to think about this decision but since I went to Watford Girls and I had a lot of friends who went to St Doms, I thought maybe I could help a little - but haha, what I have to say may be useless but you can decide
There are many pros to St Doms and the main one is the freedom. I was at Watford since year 7, so I was used to the style of it, but one thing a lot of the news girls said who came in for Sixth Form said that it was too restrictive. Like you can't leave during frees and there's a uniform (although I preferred that as thinking up things to wear everyday for Uni is hell) and also the teachers expect you to work. Yeah, they're a bit more lax in Sixth Form, but they still treat you less like adults than in St Doms. So if you want more responsibility with your education, Doms would be better. They also are more open to other choices at St Doms, whereas at Watford, it's kind of a given that you're going to Uni. Yeah, they talk a little about other options, but pretty much everyone leaves with a Uni place or a few take a gap year. I don't know anyone in my year who isn't at Uni or on a gap year (I just finished last year).
Watford does have pros too though - the teachers are incredibly dedicated and from what I've heard, the teaching is of a higher standard. Some girls left to go to St Doms and then complained to us who stayed that the teaching there was just not as good a quality and that the teachers weren't as keen about their subjects, whereas at Watford, teachers will happily spend hours of their time, giving you extra tuition just because they want you to do well, not for extra pay. Like my Maths teacher used to teach us Uni Maths after School if we wanted to go and it was all in his free time. Of course, this may not be true about the teaching standard at Doms, but it's just what I've heard. Watford also tends to be a better environment to study in - we all used to help each other do well and motivate each other, whereas my friends at other Sixth Forms, such as Doms, generally found that people tended to be more selfish with helping each other. All of the new girls also blended in really well, especially as we do lots of crazy events that kind of band people together with a whole day dedicated to 'Disney Day' or 'Harry Potter Day', so haha, as it's connected to the main School, there are a lot of childish but really fun events that go on all year round. The only thing is, there is a limit - like Muck Up Day was quite controlled and I'm sure that would be more fun at Doms.
So yeah, Watford has generally a better reputation and a better teaching standard but Doms trains you more for life with a more adult attitude to their students and also it's mixed, which is something that a lot of people prefer over a single-sex School. So yeah, tough choice xD But really, both are good and you should do well wherever you go if you're determined enough