Personally, chestnut brown is more natural, but different brands of potions will have slight color difference even if they are the same color. When you say safe, do you mean less harm to your body and hair? The fixation effect of small irritant potions is worse than that of large irritant potions, and it is relatively easy to fade, but I would rather use the one that is easy to fade than the one that is less irritating.
I wish you young and beautiful!