Both have strength & weaknesses... 12mm is the widest RF lens available, while application is kind of limited as edges shows some level of distortion. Best used for architecture and interior where dramatic effect is acceptable. The lens produces exceptional interesting & fun images in the hands of creative people. Best used with tripods and with small aperture.
15mm has more applications. Shooting kids, indoors, people is more acceptable. A great street lens, just aim and shoot. Easy to use and a pretty sharp lens (set at 2m and f5.6 almost everything is shart). Corner distortion is minimal (not as evident as 12mm). Load a ISO800 colour film, perfect for indoor shoot. I shot my wife delivery process with the 15mm and am amazed with the result. You can take a look at the pictures if you take the blood and gore.
Comparing with the Leitz glasses, my CV lenses are sooooo much more affordable! :bsmilie: