Carl, I’ve experimented with the Sammy 14mm (a mate has one) and it’s horrible. They have an long standing issue with QC, so his may be duff. Make sure if you get one you can return it once you’ve tried it
Conversely, the Sammy 12mm I have for Fujifilm X is a ripper! A good dose of coma in the corners, but for what it swallows it’s pretty decent. And cheap.
What I’d suggest is consider what else you’ll use it for. Going so wide, once you’ve done the MW once, that’s your lot. A 40mm Sigma like JA recommended is a lot more versatile.