When using a cardio machine, do you think distance or resistance level is more important? Is it better to go faster and further on a lower level, or to go slower and not as far on a higher level (calories burned and time are constant)?

I know there really isn't a "right" answer, but what is your opinion?

Short answer: Neither... in between the two extremes is usually best.

Long answer: Generally speaking (since there's kind of too many variables such as workout length and machine used, etc.), there should be a nice middle ground, neither extreme is the best. For example, when riding a bike, if you're slowly cranking at 60 rpms or less, you're hurting your knees and making your muscles take most of the hit and not your lungs and heart. This is not what you are after in a cardio workout. On the other extreme, if you're flailing away at 120 rpms or more, unless you are Lance Armstrong and have perfect form and ride 6 hours a day, you cannot actually pump out much work, so your legs are moving fast but your actual workload and power generated is really low. Most people try to shoot for about 90 rpms, with a sweet spot between 80 and 100 rpms for most people, most of the time. This is the best compromise, as you are putting most of the workload on your cardiovascular system, and it's slow enough so that you can actually pump out alot of power, which is what is needed to actually work your cardio system. Other machines and workouts should also have a sweet spot in this same manner. Basically do whatever makes your heart rate and breathing rate elevate the most.

I did wonder this. As you said, the calories burnt are the same, so I came to the conclusion that it can't make a lot of difference weight-loss-wise.

Personally I usually tend to alternate a bit, I find the workout easier to keep up that way. (Same as I tend to move from machine to machine then come back round, rather than spending a long time on each just once.)