Cardio question, harder or longer?

solman66

When I started exercising the treadmill/running was killer on my knees and gave me plantar fasciitis so I did the elliptical. I started slowly at first until I build up some endurance. I kept adding more resistance and working longer making sure I felt like I was still getting a good workout.

Recently I decided to give running a shot again. What I found out is that my knees no longer hurt, but I get winded a lot faster. On the elliptical I can "run" for 30 minutes straight at 6+ mph. On the treadmill I couldn't do more than 1 minute at 6mph.
I'm slowly starting to build that endurance up, but I guess I'm just wondering which is "better".

Right now I try to do 15 minutes on the treadmill a few days a week. Not a true HIIT, but close. Light running until I'm out of breath (usually 1-2 minutes) then walking until my heart rate drops. I'll usually do some strength training afterwards and do another 15 minutes of high inclined walking or light jog on the elliptical to finish the workout.

When I work the elliptical I either do 30 minutes of running + 15 minutes of jogging/cooldown, or 20-25 minutes of HIIT and 5 minutes of cooldown depending on how much time I have.

I would assume I'm going to burn more calories from the longer workout, but burning calories isn't everything. Should I keep mixing it up or concentrate on the treadmill since it seems to push me harder?