The 15mm starts stretching things towards the edges of the frame so I prefer the 28mm for its more natural rendering of the scene. Based on the images you've presented the 28mm appears to have a higher level of detail/clarity as well. Also the 28mm, when shot wide open, starts takes on a little bit of that large format film look with the wide-ish angle of view and shallow depth of field, as seen with your first picture of the color thread.