OK, let me try. Given that the € is worth more than the $, and if you're eating on the French side, and if the restaurant is offering €1=$1 (or $1=1€, it's the same thing, LOL) pricing for cash, then pay them in cash $ if you can. You'll come out better than 10% ahead.

I always have both $ and € cash in my pocket, as well as zero-fee plastic. I understand exchange rates, I understand 1:1 offers, and I pay in whichever manner is most advantageous to me.