To burn off the most amount of calories, I would say increase the duration and the difficulty...
If it's a choice of one or the other, there are various schools of thought on it... One being that if you keep a lower level of difficulty over a sustained time, you will be in the "fat loss zone", another being that you burn more calories with higher difficulty, and you still reap the benefits of having your metabolism raised afterwards, which has shown in certain studies, to be raised longer than if it's at a lower difficulty, the third one (that I know about) is to do High Intensity Interval Training (HIIT), where you fluctuate between high resistance and low resistance, so if you go at 90% for 5min, then slow down and catch your breath for 2min, then repeat, that you will burn off the most calories/fat/and have the benefits over the longest period of time.
I personally love HIIT on machines. It's easy to track your time, and you can mix things up a bit to kill the boredom. Like I said though, I feel like going for a longer duration is more important... You should be pushing yourself in the gym, I mean, if you're putting in the time, and being there, then you should be making the most of it. I still feel like girls who can workout and keep their makeup perfect are just not working hard enough.
