Hello! Happy to help if I can. DS1 is in 7. Klasse now.
We decided on Gymnasium and have been really happy with it with the only bump being that German schooling at secondary level is SO different to the UK system I have felt a bit out of my depth (just starting my own thread on that.)
My reason in the end was that when I spoke to his class teacher, she explained to me that a lot of the Gesamtschule used to be Hauptrealschule/Werkrealschule (ie the least academic stream) and as a result a lot of the locals will still have that kind of perception, social class-wise, of the school, and that affects the demographic that go there. Also, that although it's theoretically possible to study the Abitur the problem (for some children) is that there is no automatic expectation that they will, and the demographics of the class will also make a difference here.
I know that's not PC on MN to be put off a school "because of the social class of the pupils" and if I'm honest this whole aspect of the tiered school system in Germany bothers me a lot. But in reality we were having quite a lot of problems in the last year of Grundschule with DS1 spending time with a couple of children who were really bad influences on him and I was really concerned that this was likely to continue into secondary, and while it wasn't great that at 9 he was wanting to listen to offensive rap music, act out in class, look at drugs/nudes/etc on instagram and hang out in a park eating crisps those are really small fry compared to the kinds of trouble teenagers can potentially get into.
I made an assumption (which turned out to be accurate) that children recommended to/entered for gymnasium would tend towards being the more academically minded, who were more likely to do their homework and less likely to sneer at each other for doing well in class and this paid off for us. And happily it is not just a middle class island as I feared, there are children from all different income groups, but there is a lot less of the smoking/graffiti/machismo than my mental image of the teen years (which I know, have not yet really begun.)
I worried about the pressure, because we only have one G9 school in our area and it was really oversubscribed and far away so he is at a G8, but so far, any pressure has actually been really good for him and he doesn't stress about school work at all - to a fault TBH. :o
Also, I don't know if it's the same in all Bundeslander, but here, you get three school choices and you CANNOT mix school types. So you have to apply for 3x Gesamtschule or 3x Gymnasien, you can't mix and match. This has been lamented a lot in our local group but it seems to be to do with the place allocation.