Yes, of course it's worth it. The only reason it wouldn't be is if you are never going to stay at a Marriott.

After 15 stays you will be a silver. But early on I would ask for the .the manager, introduce yourself, and tell him that you are booked for 25 nights with them... and ask at the same time if there is a chance of an upgrade to concierge level. You'll probably get it.

If you are fairly certain you won't be staying at Marriotts beyond the 25 nights, then choose airline miles. But as you get beyond 25 nights, the value of Marriott rewards available to you will rise. Fairfield, Courtyard, Residence Inn, and Renaisance stays will also count toward your elite status, which with 50 stays will become gold.