Have not seen either irl yet but from the images here Bleu Mineral looks more interesting than Gris Tarmac. It reminds me of 05 Indigo. BUT I would pick Gris Tarmac between the two as I wear my grey bags WAY more than my blue ones.
I left mine up to fate! Have really been looking between Bleu Mineral or Gris Tarmac as well for my first Balenciaga. Sale just went up on Rue La La with Gris Tarmac and had to take the opportunity for a brand new yet discounted bag! Will post pics when I receive. Bleu Mineral is gorgeous and hoping to get another bag in the future. Looking forward to seeing yours!