Well, mine is somewhat of a hybrid of the two. I have always been automatically signed up for the 50k points, but the last few times I have been able to change to the free nights. Knowing that my schedule is scaling back (by choice) I call Marriott Rewards Customer Service before the promotion starts (but after I am already automatically registered) and some type of note is attached to my account so that after the promotion ends I call back, explain the situation to them again and get the two free night certificates on my MR account. It's actually more trouble than I'd prefer, but once they have registered me for the points, they won't/can't change the preference until after the promotion is over. So right now I'm just waiting until May 1st to call back and get my two night certificates.
Nuhusker, I had to do the same thing because we weren't going to come close to the 20 nights during this promotion. The platinum customer service rep told me to call back if the nights had not been posted within a couple of weeks after the promotion is over. Last time I had to change the promotion, the nights were posted within a couple of days. I plan to use them this summer.