In terms of cardio, I tend to do both. I mean, I love the intensity in a good spin class, but I can't go to one every day (I could, but I'd probably have to give up things like, oh I don't know, using me legs for any other purpose
). Sometimes I like a nice long, easy run. Sometimes I only have 1/2 hour and blast it out with intervals on the treadmil. It really depends.
Although, I usually want to get the most out of my time, so I'd probably vote for the shorter, more intense workout over the longer, easier one. However, intervals are a great way to get the best of both worlds. Ramp up the intensity (or incline, or resistance, etc.- you can do this on any cardio machine) for a few minutes, then back off and recover for a few minutes. Burns tons of calories in a shorter amount of time than the slow-and-steady approach and is also training your body in a different way. Intervals are part of my cardio usually 3 days a week.