it's highly likely that a Portland - Carribean ticket will cost much more than $250 r/t (unless you're flying to Puerto Rico on a charter!), so that's one point to consider (of course, you still need to be flexible to get the award seats). The other more important question is do you think you will ever need to fly anywhere in the Delta system on very short notice. If so, then save the miles and buy the ticket for $250.