It seems to be random, but there are some trends: whenever a new movie is released they tend to release a batch of new rewards that relate to that movie (eg. when Beauty and the Beast came out on blu-ray they released a bunch of BatB themed rewards, same with Dumbo coming out next week). So if you have a favourite movie and it's releasing soon, if you wait until then you should find something you like.
They sometimes post notices of new rewards on their Facebook page, so I keep an eye out for those so I can jump in on a reward before it goes out of stock. Or you can simply check the website once a week, I do that when I remember ^^;
For 2500 points, I'd hold onto them for a little while longer if you can. DMR usually has good rewards around the holiday season (like special ornaments), as well as reward promos around that time (like certain rewards going for less points than normal), so it might be worth holding onto them.

