I don't think the difference in game engine would amount to a large difference in development time. Using custom tooling and third party software each has its own advantages and disadvantages.
A lot of people seem to forget though, that development on FFXV started over from scratch several years ago. It's only about 3 years in development, not much different from Kingdom Hearts III. I also think the amount of assets produced for FFXV is much higher than KH3, with KH3 being a more stylized game, so it's not worth saying the two games take the same amount of resources/time anyway.
As for progress, Kingdom Hearts III looked very rough in the E3 2015 footage, kinda how the FFXV(ersus) 2011 trailer looked: coming along but still needing a lot of polishing. Same goes for FFXV really, it's clear from some of Tabata's comments and non-Duscae footage that there's still a lot that needs to be done.
I have no clue where both games are, both are stil pretty much "Now in Development [Hell]". Honestly I think FFXV is much more closer to completion than KH3, but they could as well be equally on the same path. As long as both games end up in good shape, I don't mind what comes first.