I don't think you should be 'torn'. If your goal is to take some photos with 'the fisheye look' then get the 15mm fisheye. If your goal is a 'standard ultrawide angle' then get the 17-40mm. Converting fishsye to rectilinear results in substantial quality loss, especially corner softness. You stated, "I want the images to still be of good enough quality to submit to high end stock sites." - I think that pretty much rules out rectilinear correction of fisheye images.