Register (anyway - if you are going to use it or not) ALSO for Marriott miles (and rewards).

Then, whatevever the length (and $ spent) of your stay you decide (you can still change your decision at check-out-time) if you prefer miles or points for that specific stay.

Rewards and Miles are "joint"-accounts (so the number of nights of both programs are counted together for future or renewing status with Marriott).

Miles: you get 500 Miles (into your preferred airline-program + after every 5th "miles"-stay/12-months with Marriott a bonus of 2'500 miles which brings the mileage per stay to 1'000). And Marriotts "pays" miles (and bonus-miles) for ANY discounted rate too!

For short-stays (and low $-bills) I choose miles - for longer stays rewards.