Ok I vote for running. People love ET because they are so easy on your body. Running is far more stressful and it's the stress on your body that makes it stronger. The jarring action also makes your arms, back etc stronger too; not just your legs. Furthermore, while road riding is extremely low impact mnt biking can have a significant impact component.

The people on the E-trainers are normally on them because they can't run. Why can't they run? It’s because they are weak from avoiding stressing their bodies. They then try running one day, hurt themselves or end up really sore, and go back to the easy elliptical trainer.

Lastly mnt biking is the best training for mnt biking.