I went to an all-girls school and this wasn't my experience all. Yes, there were some disruptions and fall-outs (I'd worry if that were never happening, when you have groups of teenagers, whichever sex.)
But overall, it was a supportive environment. There were probably some girls who didn't enjoy it - when there are 100s of people together, it would be odd if everyone all had the same experience. But there are also quite a few of us who look back on those times fondly (going by FB,) and I'm still good friends with people I met there (we're now in our 50s.)
I think this means the school itself is what's important - someone (parent or child) might prefer single sex or mixed, but I'd have thought a good school that suits the child (and what suits one child may not suit the next,) would be better than sticking blindly to a principle. Not that that helps when, as in the OP's case, both possible choices are good.
Mind you, if you grew up where I did - smallish town with a large rural catchment area - there wasn't really a choice. There was (and is) only one secondary, so the other options are to go private or have to drive some miles away to another town. School buses from the villages only went to the one secondary in their catchment area.