Excluding weight, do you think the X-T1 is a better choice than a K-3ii?
I do. It's an excellent camera that just keeps on improving thanks to major firmware updates. The lens selection for it is excellent as well, and includes some great options for astro-photography. If you need a "modest" wide-angle, the 16mm f/1.4 would be the best option available. If you need anything wider, you can go with the 14mm f/2.8, or the 12mm f/2.8 Zeiss Touit lens, or the manual 12mm f/2 lens from Rokinon/Samyang.
And let's say I go with the Pentax, do you think it would be wiser to get a K-3ii or a K-3 and put the money saved into better glass?
If you don't see a need for the pixel-shift technology that Pentax introduced with the K-3 II, which has some benefits for image quality, then I'd say go with the K-3. If you find you need the ASTROTRACER feature that iolair wrote about, you can add the $189 O-GPS1 hotshoe-mounted accessory.
Ricoh had to recall a lot of the Pentax K-3 II models, because they had some issue when turning the camera off—parts of it would stay on until the battery was removed. The K-3 is so similar, that it would simply be a safer bet to go with the tried-and-true camera.
Pentax have years of old lenses in their back catalog. It may be easier and cheaper to get even an old wide angle prime to suit your style of shooting.
Those older lenses were all made for 35mm film, so ultra-wide-angle lenses aren't all that wide on the APS-C sensor-equipped DSLRs. These are the ultra-wide-angle offerings from Pentax:
- 14mm f/2.8—cheaper than the Fuji 14mm f/2.8 by around $100–$200, but I don't know how good it is. Worth taking a look.
- 15mm f/4 Limited—f/4 . . .
- 12-24mm f/4—again, f/4 . . .
For the Fuji X system, there is a 16mm f/1.4 lens, and if that isn't wide enough, there are several options with shorter focal lengths at f/2.8 or better.
If you want to shoot specifically astro, definitely the K-3ii. With GPS built in, it has a special trick up its sleeve: it can shift the sensor in synch with the earth's rotation to get exposures of up to 5 minutes without star trails - giving the potential for amazing images of the milky way or including fainter stars - or just using lower, cleaner ISOs for night sky shots.
That does seem like a great solution, although with fast lenses one may not need that. An f/2.8 lens and ISO 1600–3200 on either a Fuji X-T1 or a Pentax K-3 / K-3 II should give very good results in most conditions, sticking to the "500 rule."