Happened to call Mak on Friday and was quickly briefed on the 2019 models. They are moving to generic (e.g., Thermoworks, etc.) K-Type probes which can be moved around inside the unit & are less expensive to replace. The probes will connect to an upgraded Pellet Boss providing better temperature control. Evidently, upgrade kits will be made available for older units. They are also adding two more top rack slots, one above and one below the existing position. (Sounded like this is mainly to accommodate jerky and such.) He also mentioned something about the hopper lid and a bracket being re-designed to help preclude pellet moisture contamination in high-humidity environments, but I didn't pay too much attention since that's not an issue around here. The price will increase $100.
If you don't think you'll need the additional racks and aren't concerned about pellet moisture contamination, at $600 less than a 2019, a 2018 model may very well suit your needs. Temp control on my 2017 is more than adequate and I use a Thermoworks Smoke to monitor grate & meat temps anyway. Might suggest checking in with Mak to better clarify the differences.