Here is a write up from Lonely Speck that honestly is fairly complete in the information to help you identify the lenses most suitable for Astro Photography.
To more specifically answer your question, in terms between the two Rokinon lenses, they would both be suitable lenses for what you are looking to do. The wider lenses will exhibit a lot more lens distortion, so make sure that is something you want in your artistic vision of your composition. A certain degree of this can be fixed via post, but 100% of it will not be able to be removed, especially more exaggerated distortion in a 10mm lens. As for the F/ stop, you are really only losing 1 stop of light going from the 16mm to 10mm. With cameras these days and the ISO performance, this really shouldn't impact the end image much.
In regards to super wide angles for landscapes, I personally consider anything wider than 24mm on a full frame camera super wide (wider than 18mm on APS-C). Better for landscape is really subjective and really what you are looking to do with the image. Super wides are great for landscape as it allows for a larger field of view, but does come with some required composition knowledge to make moving images. If you shoot with a ultra wide lens, generally you want a strong foreground object or your image will look rather flat and boring.
Here is an example of a photo I captured at Mystery Island in Vanuatu with a 21mm F/2.8. I used the tree in the foreground to frame the landscape. If I didn't have a foreground element, the photo would not be as moving.
Here is another image I shot in Fiji without a foreground image, while still a decent shot, not quite as moving because of the lack of a strong foreground element.
Now below here is a photo I shot with a 35mm - which the composition of this image is a bit better without a strong foreground element.
For astrophotography, the benefits of the Tokina lens is not as big of a benefit. Really the best things you will get from Tokina is the ability to autofocus (which with astrophotography is rather pointless) and their better QC (Odds of getting a bad lens are lower). Now with that, if you plan to do other photography other than astrophotography, you gain the flexibility of having the auto-focus which makes other types of photography easier. Landscapes, not really as important, but for say portrait photography, would make life easier.
Shot with a Tamron 15-30mm F/2.8
Another thing to ensure you check out when doing astrophotography is this exposure calculator based upon your lens, aperture - which then tells your your ISO and exposure speed to ensure you don't get star trails.