Someone just asked this in Ask & Answer. Good question. I was signed up (against my wishes) for the "stay 25 nights" and get 40,000 points, but I called to say I didn't want that bonus but the 2 for 1. Well, I checked and not only has my anniversary certificate (as of last month) not posted, but I have not gotten credit for 2 stays. And it still says I am signed up for the 25 nights bonus. Sigh.
I called to change to the 2 for 1 as well. I was told at the time that they can't change the computer once its in there so it's noted in my file. I was told when the stays were achieved to call them back. I'm about to give them a call...I'll let you know if it worked!!
Marriott, in the future give us the choice and allow us to sign up for the promo that will fit us best. Perhaps a 30 day time period to choose then it defaults to your choice.