I don't have any science to back up what I'm saying, of course, but I tend to think this is a matter of personal preference. Sure, different machines work your body in different ways, but if we're talking about cardio machines that you're using primarily for the calorie-burning benefit, it's all about your heart rate. If you're used to the machines and you know just how to get your heart rate up (what intensity, what speed, etc) on each of the machines, then I'd say what matters is how long you worked out with an elevated heart rate OVERALL. Some people would rather have their arm hairs pulled out one by one than stay on the same machine for more than 30 minutes, because it bores them, and others like to stay on a machine long enough to get into a groove. I fall somewhere in between - it depends on my mood - though I tend to stay with one machine for about 40 minutes or longer. I always mix it up, though, with intervals of hills or speed or resistance so that I'm being challenged.
Anyway, bottom line: I think what counts is how much cardio you're doing at an increased heart rate, because that's what burns calories. However you burn those calories is up to your personal preference. They do say, however, that your body gets used to movements and that challenging new muscles by hopping on a different machine can kickstart your calorie burning again. So I suppose an hour on the elliptical every day for the rest of your life might bore your body into some kind of rebellion!
But I'm sure some of the moderators or other posters will have a much better and clearer answer for this!!
Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible. -- St. Francis of Assisi