Wow, how did I miss this thread? A lot of interesting opinions here. Really good discussion from clever people.
Just one point that seems to have been completely missed here though; how hard are you exercising?
This is one thing I find slightly bemusing when you look at the differences between athletes and bodybuilders/fitness hobbyists. I see some people doing HIIT and it doesn't look like they're going hard enough to call it HIIT, I see some fat people trying to get lean by walking for an hour, and I see some extremely lean and extremely fit runners/triathletes/cyclists/competitive sportsmen doing "steady-state cardio" but hard. All of it will be effective, but what's going to burn more calories, an hour's walk or an hour's run? 20min of half-hearted intervals on the rowing machine with terrible form, or 20min of fast interval sprints done perfectly? How far are you going? How much time did that distance take? These are the questions I'd want to know.
HIIT vs Steady-state is nowhere near as black and white as you guys make out because they might have totally different meanings to people with different ability levels.
So here's where I "step outside the box" with my opinion: what if you ignore calories, ignore optimal training and think about getting fitter. If you were primarily interested in functionality, wouldn't you just do a mixture, because that would be the best thing to get fitter?
I realize this thread is mostly about aesthetics, so I'm kind of coming off a tangent here, but it is strange that this hasn't even been touched upon. I mean when comparing steady-state to HIIT, and saying HIIT burns more calories, how fast is the steady-state being done? A practical experiment would be to randomly pick fifteen people from the gym, put five on bikes, five on rowers and five on treadmills and tell them to do 30 mins at a moderately hard pace and ignore each other. The difference between distances travelled, calorie expenditure and exertion (because some people naturally put more effort in) will be wildly different.
It's not so easy to compare cardio as this thread suggests. Obviously Allen is the man to listen to when wanting to get lean, but I sometimes find it frustrating that people only see cardiovascular exercise as a tool instead of something to try and improve at in the same way you would with strength training. It's also what I like about crossfit and its no surprise at all that the best cross-fitters are extremely lean without even trying to lose fat.