"Least Useful Response Award" nominee.
I feel your pain. I will be in a similar position soon. I'll have to decide between a used X, new Y, or the CT (if available when we get back to the states).
The X does have that "wow" factor that a lot of people like, whereas the Y, while still beautiful, is less showy.
You didn't mention what AP suite is on the X you're looking at, but I'm assuming a 2016 probably has AP 2.0 or lower. This would limit some features (no sentry mode) and possible FSD features.
The other big consideration is the battery tech and range. That P90X is probably rated at 250 miles. That would be the assumption with a 100% healthy battery, which I wouldn't assume on a 4 year old car. Y will get you 315-316 new, as long as you don't add the PUP. Plus the newer battery tech on the Y will charge much faster than the X (which may also be throttled due to age/degradation).
X will come with free supercharging. The Y will not. Might be a factor if you are going to do a LOT of traveling.
X has been in production for years now, whereas the Y is brand new and they are still working out the kinks (Sandy Munro recently showed how Tesla used a 3D printed part on the Y he's tearing down, because they fixed a flaw and didn't want to delay production).
Just a few things to consider.