The difference is: there is no more expensive iPhone that doesn’t have FaceID. As long as the Air-line doesn’t have it, I’d say there is a zero chance of the base iPad getting it. Unless they kill off the Air 3 or update it at the same time. Both seek highly, highly unlikely to me. It makes much more sense for this new iPad to be the last one with Touch ID. Then the Air 4 will get FaceID in 2020 and the base iPad in 2021.
Nah.
This is ancient history, but nonetheless it sticks out in my mind, and is a relevant precedent. I had a 2001 12" iBook G3 600, but I found it too slow, mainly because it did not have AltiVec. Plus it only had USB 1 and combo drive. I desperately wanted a 12" iBook G4 with USB 2 and SuperDrive. In November of 2002, Apple released a PowerBook G4 1 GHz Titanium 15" with SuperDrive, but only with USB 1. At the same time, Apple launched another iBook G3 again with USB 1, and with combo drive.
I was disappointed I didn't get my 12" iBook G4 USB 2 with SuperDrive. However, I needed that G4 with SuperDrive, so I bought the 15" G4 with USB 1.
Just
two months later, Apple launched a smaller and cheaper 12" PowerBook G4 with USB 2, as well as a 17" model. This 12" was my perfect machine, but I was stuck with this USB 1 model, in the 15" size I didn't want. Furthermore, Apple didn't update the 15" to USB 2 until 9 months later.
This new 10.5" iPad Air could be the 15" G4 PowerBook Titanium of 2019, the last hurrah with a legacy interface component (Touch ID), while newer cheaper models get a more advanced feature (Face ID).
However, a 10.2" iPad with Face ID would get generation 1 Face ID. Generation 2 Face ID would be limited to the 2019 iPad Pros.