I feel for you, it's not an easy decision, and really does depend on the child, the 'fit' and the alternatives. We live in SE26 and DS (y10)goes to St Olave's and DD (y8) goes to NW (we're just in the 9miles catchment PettsWoodFiona refers to).
For DS it was our standout 1st choice (despite the very negative online stuff one reads everywhere). He's a very academic boy, we hoped it would be a good fit, and were very pleased he got into all the grammars we picked and were lucky enough to get 1st choice. He does not mind the commute (about 50 min door to door, 24 min on the train and walks either side) really, has a group of 'train' friends, even with the odd blip over the years, have never ever regretted our choice.
DD has been another kettle of fish. Clever girl, top table, but not as 'bookish'. The good local girls comp 20 min walk away is new and shiny and on the up. Alllllllll her friends have gone there. I was very conflicted about which school to put at no 1, and in a way slightly hoped she would not pass the entrabce exam for NW so we would not have to make a choice. She passed, was thrilled is doing well, settled and happy - and yet. And yet. She finds the commute much more tiring than her brother, and she finds the pace and the pressure much more upsetting than her brother at the much more evil St Olave's.

I think she might have felt more chilled (and more rested!) going to the local school 20 minutes away - but we only get one shot, and we made the best decision we could with the best of intentions, taking what she wanted into account (she really wanted to go to NW).
So 2 children, same commute, similar schools after same south London state primary and very different experiences mainly based on who they are rather on what the schools are like, I think. Both schools have been absolutely lovely, and effective whenever I have been in touch regarding any pastoral issue, and that's the most important thing to me.
Good luck with your choices!