Two things that are true about 6T but not the 9/12:
It was pre-OS merge and things still felt more like vanilla Android. I still recommend it as a barebones Android experience, but if I'm asked whether there is bloat now ... I have to be honest and say that there's been persistent creep in recent years.
Form factor. The smaller size sits better in the hand and pocket IMO, and I don't recall feeling (after having OnePlus 5) like I'd lost screen space.
The form factor of the 13T is what's appealing to me, plus the insane battery capacity. I feel like the 6T is already too large so I'd hate to get anything bigger.