Hi LostInSp_ce,
I've just been through the same exercise, albeit for a full frame Nikon D600.
My main area of interest is nightscapes featuring the Milky Way and a terrestrial subject. I've been using a Samyang 14mm f2.8 with pleasing results but my tripod had a little whoopsie at a cemetery one night (bloody ghosts
) and damaged the lens.
In my search for a replacement I narrowed it down to the Sigma 14mm f1.8 and the Tamron 15-30 f2.8 for it's versatility. With your camera's 1.6 crop factor these are both a bit narrow for getting most of the arc of the Milky Way without stitching.
While I was getting my funds together I continued searching and came across the Samyang 12mm f2.8 and checked some reviews, lots actually, and most were very positive.
It then occurred to me that for the price of the two above mentioned lens I could get the Samyang f2.8
and a SkyWatcher Star Adventurer bundle ,which is the way I've gone. Of course since I picked up the Star Adventurer it's been overcast so I haven't had a chance to test it outside. The Samyang is on it's way from Hong Kong and I should be able to give some feedback on it early next week.
A little while back I decided that Milky Way and nebula photography was going to be my 'thing' and to that end I got a full frame camera, firstly for the wider FOV, and also for the better light gathering ability the bigger pixels offered. Have a look here for a comparison of the sensors of the 70D and the 6D.
https://www.digicamdb.com/compare/ca...-canon_eos-6d/
I don't know how keen you are on wide field astrophotography, but now is the time to give some serious consideration to a switch to full frame, something like the Canon 6D, before you lock yourself further into the crop sensor scenario.
If you stick with the 70D I think that the Samyang 10mm f2.8 is a good option to minimise stitching, giving you the FOV of a 16mm lens.
I also came across this handy plug-in for Photoshop CC and Lightroom to minimise the fisheye effect.
https://imadio.com/products/prodpage_hemi.aspx
My reply has sort of gone off on a tangent to your original question, but it is something that I think deserves your consideration.
Good luck with whichever way you decide to go.