While both muesli and granola are a mix of grains, seeds, nuts and dried fruits, the main difference is that granola is sweetened with honey or maple syrup and is baked. Muesli is not baked.
They each are regarded as a healthy breakfast.
I have to say I'm on a real porridge kick lately, after discovering and feeling first-hand the benefits of buckwheat grains. By "activating" the grains overnight, it provides a lovely nutty flavour thats high in amino acids, great for your tummy and serotonin levels, meaning it actually does make you feel full and happy!