Whoever is making your ring should be able to give you a quote for both metals.
I would look at the two metals and see which one you prefer. I don't know much about yellow gold, but I wear a 14K rose gold band with my 18K rose gold diamond ring, and there's a big enough difference, visually, that I *really* prefer the 18K rose gold.
Both will have a different look. The 18Kt. is a purer metal(75% Gold), so it will have a brighter yellow look, but it will also be softer for the same reason. Therefore, it will scratch easier. That being said, I would prefer the 18Kt. Shank, with its "richer" look.