I like # 15 better. The breed of cows that we milk are Holsteins - Black and White, so the head should resemble a Holstein cow. I'm not sure if a side profile or head on profile would look better, though. Also, no horns.
I like # 18, however the cow looks more like a beef cow than a Holstein dairy cow. We have pictures of Holsteins at Lambfarmsinc.net or Oakfieldcornersdairy.com. The top of the head is too broad.
# 27 - nice job - another suggestion to change the marking on the cows' head - Holstein cows are typically back on the sides of their faces and have a white stripe from the top of their head down to their nose