From my experience, buying an inventory MX was by far the best option. My story is summarized here:
2017 VIN Model X Order and Delivery
I used EV-CPO.com from
@HankLloydRight and found exactly our desired configuration in an existing inventory car. It had the newest VIN number I saw on the site at that time (plus it was a palindrome), and included all the latest options which you can check via that site (or via the Tesla link cited from which you can scrape all the options). At that time it was AP2.5, plus PUP features had migrated to standard, was uncorked, etc. Strangely, the car had a significant discount but was listed as a new “50” miles. Using the price histogram on the site then (and even after I bought it), it was the absolute lowest price sold for those options. I went to Tesla and asked them to confirm the options, and learned that the car was still on the assembly line! Placed the order (at that time there was also the 0.99% financing promotion) and picked up directly at factory with no delivery charge applied. There were a few snags, like a missing tow package and the adaptive LED headlights, but Tesla took care of these either before final delivery (lights) or later at Service center (tow package). In addition they later added the HEPA filter and biodefense mode at no charge when I asked to pay for an upgrade. It was a great experience and is an awesome car.
If I want to be critical, I would say the only thing that could have made it incrementally better was to opt for a larger battery, as mentioned above. But practically it would have only made a difference for 1 trip in 1.5 years. With young kids on a long trip we are stopping more often for supercharging than a larger battery would afford anyway. So think about your usage scenarios. For us it was the right choice. We also removed autopilot before purchase to minimize current and future registration costs. Now it is clear that was the right decision given the drastic discount on autopilot software, but it could have gone the other way. (We just purchased the autopilot on fire sale before price increase promised on Monday).