I try to do incline only because jogging or running is too hard on my knees. But honestly, to get the same calorie burn, I have to walk at a really high incline, and doing that is SO challenging. For some reason, it "feels" harder to walk at a steep incline than to run.
I've been told that walking at an incline helps to build more muscle, but I'm not sure about that.
Depends what your goals and needs are too. More steep of an incline builds muscle - especially legs and butt. No incline and faster speed is for cardio. I typically do both in a session - some slower walking with steep incline and then some running.
"I'm through accepting limits, cause someone says they're so. Some things I cannot change but 'til I try I'll never know!" ~ Wicked ~
"Have you ever looked fear in the face and said 'I just don't care!'" ~ Pink ~
I agree that it depends on what your goals are. A mix of both is ideal. Keeping your heart rate up is the important part.
However, I always keep my incline on a minimum of at least 1-2 when I run. This helps mimic the conditions of running outside. Since the treadmill is easier to walk on than the ground (the belt moves along to help you so there isn't as much resistance, whereas the ground doesn't so your body has to work harder to propel you along) I've always heard a small level of incline helps to try to mimic that resistance of walking outside.