Which to pick HBS or Tiffins girls? Travel no issue.

For 2020 entry? What about your daughter who was doing exams last year? Where did she get a place?
Both are very good schools that get good results partly because they select heavily at 11. Neither is "better" than the other.
Choose the one which appears to suit your daughter best when you visit, which may include location, sports, pastoral care, extra curricular activities, whether you need to move house, other children at senior or primary schools etc.
No-one on the internet can tell you that as they don't know you & your family.

ETAon re-reading: or have you been offered one of these & now offered the other from the waiting list?

What a dilemma to have just days before schools opening!

Both schools are situated very far away from the other one and from Nonsuch/Wallington/Tonbridge.

Unless one is near your home, decline both and apply for the school near your home. Your DD would thank you and you would avoid relocating/commuting long distances for 5 to 7 years.

Is TGS offering outside catchment now (by all accounts, that would be unusual)? If not, is one correct in assuming that home is within catchment for TGS and therefore wondering exactly how travel to Hampstead Garden Suburb 'isn't an issue'?

If it's deciding where to put first on one's CAF for 2020 entry, should either turn out to be an option, exam result wise, then HBS will also have a priority area by then and nowhere in the TGS priority area overlaps with it.

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

If you able to pick one of these two schools then that seems to imply you have been offered both?

The better one for your family will be the one where you do not have to move and put your family through the upheaval of moving to another area and leaving friends and home and potentially other family.

School is part of your life, it should not be your whole life.

Choose a school that fits into your life. DG

There's not enough detail to give an objective response here. If we assume this is for September 2019 entry it's hard to understand how travel cannot be an issue given the schools are quite far from each other.