If I was in the market and was looking at the 6300/6400/6500, I would choose the 6400. IBIS will make it easier to get some shots, but I don't think it will be that big of a jump if you already have OS lenses. The real perk for the 6400 is that it has Eye-AF and claims to have a slightly faster focusing time - it will (probably) do a better job of intelligently focusing on people and pets. I have the a7iii, and one of the features I love most is Eye-AF.
I think the comparison is close. You get a bigger burst buffer and IBIS with the 6500, but I think I would get more value out of the improved autofocus. There is a chance Sony makes further software improvements to the a6400 (for example, they just released a firmware update for my a7iii that brought extra features to the camera), but since the a6500 is older, there is a lower chance you'll be able to get new features via firmware update.