Hi I personally would recommend the 15mm it is wide enough and easier to get decent pictures from it, I think it is a great lens, I have made some great images, sorry I have none scanned and posted yet. The 12, it is extreme and more than you would probably need for most any shooting you will do. The other issue is the decentering -dark edges you will get with the 12, nice lens also, but the decentering bothers me every image comes with the dark edges which is to be expected. I would love to get a center ND filter for it, but they run around $250. +/- everywhere I have seen them. The best way to get decent images with the 12 is to either use black and white which forgives and works with the decentering to a decent looking image, and I find when I use the 12mm for long exposure yes night time type images 10 seconds or longer I seem to get the best results.
So really if you want an honest opinion, owning both lenses I would recommend the 15mm great images every time small lens, wide enough for anyhting, I love it. I have the spirit level which is an awesome accessory I recommend you get that with the dbl shoe also, it takes 1 min to get used to using it while composing, and you will be thankful to have it when you get your pictures back.
Roger