Hey, when I first started training, and granted I had no fitness whatsoever, I started using those for all the same reasons. And I got a good sweaty workout. But when I started to use the tread mill I found that I couldn't go for half as long. I was tired much more quickly. I think that running utilizes a lot of core muscles for stability and just to keep your fucking organs from sloshing around and is thus harder and perhaps better. In my case I want to master running, by which I mean run for a time and at a rate that I'm not totally embarassed to tell people about before I will start using other methods for cardio. but that's just my unique case.