both great lenses but I find the 12 is a bit more limited use and even the 15 has that real wideangle look which can threaten to dominate the picture
i now use a zeiss 18 and the 12 which is quite a happy combination... the 18mm is very functional & producting quite natural images and the 12 is a bit of fun and for that really wide effect
the 12 is bonkers on the M7 ....
examples of all 3 here:
Collection: GLASS