I mix it up, I do a few lighter sessions,walking at 15% incline 3-3.5mph,sometimes use the stairmill as it is relative for my job and the rest is intervals either treadmill or elliptical depending on how sore I am
Stair master if needed seams like you can really burn in less amount of time on the stair master, throw a lil sex and a posing routine in there and you'll get ripped.
Cardio can get boring it's a preference but sometimes certain types of it is needed to achieve a desired look or this can depend on special needs too.For instance if you were training to compete as a bodybuilder then running everyday may not work cause of muscle depletion. Then stationary would be best but say as a bikini or figure competitor running would be helpful to lean the muscle bellies out.
I do cardio on an encumbered indoor cycle or an indoor rowing machine. I managed 2 hours of low intensity on the indoor cycle today and burned 1018 calories.
I find that Brain Sync or iMusic helps me get into the zone quickly and that it helps me stay focus for a long time. I could easily do another hour but my arse begins to hurt so I have to stop.