I'd save them and use them when you're going to some out-of-the-way location not served by Starwood. That's the advantage Marriott has over Starwood: Marriott is everywhere! 18,000 points gets you 2 nights at a small-town Fairfield Inn - which could run you $200 if you end up having to pay.

Plus, if you want a few more points, they are easy to come by. Just signing up for the Marriott VISA card can get you 20,000 points for no cost (the annual fee is waived the first year).

Of course, keep in mind that the points may expire if you have no activity for 12 months. If you're going to lose them, then use them.

Also, if you do want to exchange them for United miles, at least wait until you're ready to redeem a United award ticket. Nothing worse than having transferred them to United, not use the United miles, and then wish you had the Marriott Points instead.