I agree with
@lisawhit
I think the hammered gold is gorgeous and much more elegant. It's a classic, and metal work is lovely.
The guilloche is super shiny metal, and to me, it gives off a bit of that disco-y reflecting vibe, especially as a necklace near the face.
If you plan to wear it as your everyday necklace, you may feel you don't want that super shiny metal vibe with, say, a t-shirt.
If the shiny metal look is what you want, then guilloche is definitely that.
I vote hammered gold. You can also get matching hammered gold earrings in the future. That would be a gorgeous set.
Good luck with your decision, and congratulations on your soon to be new piece!