I am in the same dilemma regarding the 10 motif yg guilloche. I am aware that no piece of jewelry looks as good as new and they may lose their shine, scratch or dent etc.
However, I am yet to see a tarnished 18k gold (or even 14k). Some say the alloys tarnish but I am confident that a high quality piece wouldn’t tarnish even though there are alloys mixed. I have never seen it on a Cartier piece or any gold piece (no matter how old the piece was or how often it was worn).
I contacted VCA online customer service yesterday and asked specifically about the 10 motif yellow gold guilloche piece and whether it tarnishes or not. The agent was vague about it and I didn’t get a clear answer. I have the 5 motif guilloche but I haven’t worn it much to judge (as I plan to attach it to a 10 motif so didn’t want to scratch it). Part of me is telling me to go ahead with the purchase while the other part is telling me to go back to Cartier.
My advice would be to evaluate what you are willing to put up with should it tarnish.