Well, the big difference? You are paying the additional money for the tips on the end of the exhaust pipe. I don't have either of the tips, as the '16 Platinum has a cutout for the exhaust, it just terminates a basic tip in the shroud by design.
One thing that is not explained in any of the descriptions for any of the kits, and I found out when I actually got under there and looked at it - It is not going to run all the way to the cats (unless you make it). There is an X pipe that is in place behind the catalytic converters. Look under the passenger side, you will see the parts pretty easily, the exhaust will cross in an X (or H, whatever you want to call it), then go to a resonator that looks like a muffler in the center (both pipes will pass through it) and then each pipe will go up to a muffler. The core on the muffler is 2.5" on the '16 kit - the pipes are 2.25".
On the '18/'19 specific kits, it also has the 2.25" pipe up to the mufflers, 2.5" muffler core, exit, and then the tips step from 2.5" to 3.5" carbon tips, or the rolled black chrome tips (I emailed Borla to confirm that this was, in fact, the design on the 18/19 kit, as well as the '16 kit).
Were it me? I would check the fit it for the '18, see how off it is, or if it needs to be adjusted. Then once you know for sure, buy the '16 kit and weld on whatever tips you want. It is much cheaper, and it will not really sound different. There is no performance advantage between the '16 and the later kits, and I would wager that the sound is going to be close to identical.
The '18/'19 Explorer, realistically, should have no difference in the design compared to the '16. The engines are still identical, the exhaust system is identical (except the tips). The transmissions are identical. I was going to consider updating to one of the newer models, until I saw that the drivetrain did not change until 2020, so I'm going to keep my '16 as long as I can.
This diagram will give you a comparison between the factory routing from the older model, if you want to compare to your own (you might be able to check a Haynes manual for the 2018 to confirm the exhaust mount points are all the same). I couldn't find a diagram for the 2018/19, but I'm going to guess that they are.
Not that it covers normal driving scenarios, etc, here's the 2018 Borla Kit in action on an Explorer Sport.