It kinda depends on your goal. If it is weight loss, conventional wisdom says that longer rides, 30 minutes or more, that get your heart rate up and build a sweat, but don't leave you panting will burn more fat. Shorter intense rides, on the other hand will tend build muscle faster than the long rides. Since you need mucscle to endure the longer rides, maybe mixing it up is the key.
Hopefully 1 or 2 of our "in house experts" will weigh in. I'm mostly speaking from experience.
2.5 years, 45 pounds later... 128ish pounds