Calories burned = work done. Whichever required more work will have burned more calories and odds are good that it was the 25 miler.
Here's a short loop around my neighborhood that I did at a pretty good clip (not fast but working hard)
http://www.strava.com/activities/116400956 and that works out to 512 calories for 8.8 miles. Avg Hr 143
Same general route with my son the preceding Monday (minus the big hill)
http://www.strava.com/activities/114124409 and it was only 310 calories. Avg HR was 111, so I was barely working. (I have a power meter so those numbers are pretty reliable)
Almost the exact same length of time between the two rides, and granted, they were short, but I think it's pretty clear that if I went for an easy ride that was twice as long, it would burn more calories, but not 2x.
The answer is clear - go for a hard 25 miler.