What's better, increased intensity or length?

When doing cardio is it better to have a longer work out or a more intense one?

Better for what?

''better'' is relative.

personally, i try to increase the length first and then work on steadily increasing intensity at that length. I've gone from 30 minute so-so elliptical sessions to pretty intense 60 minute sessions that way. Because of bad knees, i only road run once a week and i've seen great progress in my road running because of the elliptical work. I don't see me spending more than an hour on cardio going forward, so it is a focus on intensity now.

i'd say length

True, your goal will dictate was is "better" For weightloss, shorter interval training seems best. High intensity followed by recovery followed by high intensity and repeat. Treadmill and elliptical is good for this too. For endurance, it is more a function of time spent at an elevated heartrate.

Intensity (and girth)

Oh wait...sorry...my bad ;-)

my thoughts exactly. Came in expecting filth, left disappointed.

lolwut

When doing cardio is it better to have a longer work out or a more intense one?

Depends on your objectives, but in general probably a combination.

Long duration medium intensity improves endurance, shorter duration higher intensities improves your ability to convert fuel, essentially improves your exercise efficiency and repeated short duration highest intensities improve your ability to process oxygen, increasing your VO2Max, which raises your basic metabolism.

Of course what you mean by long is probably very different to what I'd mean, and similarly medium, higher and highest intensities are probably very different for each of us. The three are very complementary, the longer sessions give you far more capacity to push your intensities higher in the shorter sessions, so I personally wouldn't advocate only doing one of those unless you have very specific goals in mind.

I can't remember where I read it, but the advice for fat-burning is to do it longer rather than more intensely.

Not quite so simple, and the whole idea of fat burning zone is quite outdated. By operating at a moderate intensity one can keep going for longer, therefore burning significantly more calories than in a shorter session. It's already upthread but I'm sure there will be a few more unthinkingly advocating HIIT as the ONE TRUE WAY(tm)

Different intensities have slightly different effects on the system, as far as where they draw their energy from, but for the average exerciser the differences are negligible.

It does really boil down to what one is trying to achieve. I'd go out on a 2-3 hour run because I'm training for endurance, the fact that I'll burn in the order of 2000 calories on one of those sessions isn't really the point. If I was focusing on burning calories I'd probably suggest a 30 minute higher intensity session as the most time efficient compromise. Highest intensity intervals burn most, but aren't sustainable for as long.

If you're training for performance, intensity might have more benefits than duration but if fat loss is your primary goal, go for length when/if you have to choose between the two in terms of your internal resources.

And again, the three modes are complementary. By including threshold and interval sessions one improves the intensity that one can sustain for longer. If one only increases time, with no effort to increase speed, then there is little progression. IF one works on both then there is greater effect.