That's a tough decision, I've gone through this myself with lenses. Normally for lenses, it's about $100 difference depending on the lens. Personally, I would go with the USA or UK if I could afford it. For the Mark III, I'f I'm spending $3,000 on a camera, I'd want to make sure it wasn't grey market. I also don't like buying used cars used equipment, if I'm going to spend my coin I want it the best condition possible. That's just me, some people don't mind used items (and I'm not saying the grey market products are). I have bought one lens used and it saved me a lot of money, so far, so good.

I'm sure that didn't help much but hopefully it'll weigh in some on your decision.