I've gone back and forth with myself over the 15 vs. the 8-15. I think the 8-15 is reasonably priced, but I don't know that I would use the wide end of it very much and question if the difference in cost could be better used somewhere else. Do you have experience with either lens? What types of situations would you want the circular image the 8mm produces on a FF body?
The zoom range of the 8-15 is nice if you have camera bodies of different formats, which you do. You can get full frame 180 diagonal AOV with both bodies, and you can decrease the AOV on the crop by using it at longer focal lengths. The longer focal length on a crop effectively uses the center of the image circle, which is less distorted.