OK, i recently added a 5D3 to my body line up, so I now own both a 7D MKII and the 5D3.
I was going to sell my Tokina 11-20 2.8 but having second thoughts. It is great on the 7D2 (effectively 17.6mm FOV) and I technically can use it as wide as 16mm on the 5D3. Sure it suffers a bit on the 5D3, but it is technically usable.
I bought a Irix 15mm 2.4 and we'll, not super impressed and very limited to its use. It's going back.
I'm thinking to buy the Sigma 35mm 1.4 Art as I see a lot of good feedback on it. While I am not well experienced in stitching, I have done it and see the benifits. Seems like a much better option than the 24mm 1.4 plus it will be a nice low lights close up lens.
At some point I could see the Sigma 14mm 1.8 being an interesting wide angle to add. This could serve for both night/daytime landscape or interesting shots of automotive, live performances etc.
Question is, which one first? Leaning towards the 35mm even though I own a Tamron 24-70 G2 which btw at first glance seems fairly good on stars at 24mm wide open. I will look more at the shots I took last friday.