if it's brand new, warranty is covered internationally for pam.
AD - more satki feel, you may be able to create relationship with the SA but by buying one watch, no use. unless you can keep intro-ing ppl to him/her.
Grey market - same watch that is sourced from overseas AD, without tax and all the other blah blah stuff, currency exchange, it can be sold cheaper.
pre-owned, check for warranty, box, papers with serial number matching to the watch case. can bring to richemont for verification if you want.
for me, i will go wherever is cheaper. lol i don't see a point having a business relationships with AD because i won't be buying watches every once a month.