Well, since you're planning to get both, I suggest purchasing the X-T20 first to see if it's lacking any function that you truly truly desire which the XT-2 can offer, then get the X-T2 to make your photography life fuller
On a more serious note, I got myself the X-T20 and was glad that I did. I live in a dry and temperate place (in Australia) so weatherproofing isn't that crucial for me really. Slow shutter speed can be countered easily by getting an ND filter (a variable one is good), which you will need one anyway even if you're using X-T2, for long shutterspeed effects during the day (e.g. flowing water) and also for filming. Lack of grip? Buy a $20 quick release grip. I have to say that the next to nonexistent grip really took me a while to get used to. Anyway I just bought the $20 grip since I've started filming more these days.
Speaking of filming... Not that you have mentioned anything about it, but if you do decide to make use of their kickass 4k function one day, I would think X-T2 is the better choice, mainly because there's an option to add a battery pack which comes with a headphone jack. Plus a bigger camera with better grip just feels a lot more steadier. You cannot imagine how much pain in the butt it was for me to look for ways around short battery life and the lack of headphone jack (for audio monitoring). This is how you can overcome these 2 problems (once again, if you ever want do filming with X-T20):
The way to overcome the battery issue is to buy a $20 dummy battery that comes with a usb cable that you can use to connect to your 20,000mah (or even bigger if you want) battery pack. All of a sudden you get 16 times your original juice! Much more juice and much much cheaper than buying a battery pack.
The way around the lack of headphone jack is to either get a mic with a headphone jack or get a voice recorder with an input for the mic and output for headphone and camera (with a splitter). The latter gives you dual recording on the camera and recorder, in case either screws up.
Just to make your choice even harder, I sometimes wish I had just gotten a Sony/Pano instead, but I'm already 2 cameras and 6 lenses deep into Fuji. It's hard to turn back. Not to say I hate it in any way! It's just that the grass is always greener on the other side
Oh and by the way, the Samyang 12mm is f/2 not 2.8. It's probably the fastest super wide angle x mount lens you can get if you're serious about shooting stars. Don't get the 10mm, it's chunky and slower.