I guess I'll keep buying the French Gros Sel de Guérande from world market, plus the fact that it tastes really good when sprinkled on meat :)
They both start with the same "raw material": water of the Northern Atlantic, which I would think is mixed enough not be significantly different in these two locations. Guérande is probably one of the largest locations for sea salt, but the processes are far away from industrial scale. They manually remove contamination that they see, but I've found a few fish in the salt. Contamination at the millimetre-scale is tested for, but there are no filtration processes in place to reduce them.
Anyway the conclusion rather place the problem, not on the consumption of those salts, but in the gradual accumulation in products from the aquatic environments.