I received the stay 6 get 3 however I sent an email requesting a switch. I got this as a reply:

"Our records show that your account is enrolled in the 2013 Spring MegaBonus Promotion. The earning dates begin February 2, 2013 and ends April 30, 2013. You have been selected to receive an electronic free night certificate, valid at hotel categories one through four, after every two paid stays. You can repeat this up to a total of six paid stays to receive a maximum of three free night certificates.

However, as you are a valued Platinum Elite member, I would be happy to extend you the courtesy of changing this bonus offer to receive a bonus of 25,000 Marriott Rewards points when you stay a total of 15 paid nights. You can receive an additional bonus of 15,000 Marriott Rewards points, up to a maximum bonus of 40,000 points, when you stay a total of 20 paid nights."

I am interested in the points since I am working towards LT Plat status.