Upload & Sell: On
There are a couple of threads on just this issue.
To summarise, they are pretty similar centrally, the 12 keeps up quality to the periphery a bit better as you would expect.
Optically, the 12 is overall better. But damn 10mm is fun. And 12 is already very extreme - not a normal ultra wide length, so why not go the whole way?
Maybe you have used extreme wides before; but if not I cannot emphasise enough how much wider these are than 21. My 10 makes my 21 look like a tele! If you just want a lens much wider than your 21, but is a more *generally* useful FOV, and likely optically even better, you may be better off with a 14 or 15.