We chose the X over the Y because we have 2 small children less than 3 years old, and it would be great to have a vehicle that can also relatively easily cart around 2 grandparents as well in one vehicle. I find that to be likely challenging in the Y, and the Y 7 seater isn't available until 2021.
The X is 11 inches longer, which is an entire class bigger (e.g. like comparing a Ford Escape to a Ford Explorer). Obviously, some people will want or need the larger vehicle for the space and capacity and some won't.
I think with 1 kid, we'd get the Y. With 2 kids, we decided the X for that additional space and to be able to haul their friends around in the future as well.
There are, of course additional reasons why we went with the X, but that is the main reason over the Y.
P.s. we take delivery of our X a week from Wednesday. I have no doubt that once that happens, a huge X price cut will be announced as that seems to be the trend when I get something new and expensive.