Major differences between breath meditation and body scan meditation?

I have done several Vipassana retreats in the past. Some questions came up to me several times without being able to answer them. - **What are the major differences between mindfulness of the body (e.g. Body scan/Vipassana meditation) and focusing on the breath (i.e Anapana Sati meditation)?** - **How can I focus my attention as it arises across my body? I am able to focus on my breath between my nose and my mouth, but when I am aware of my sensation on my legs for instance, should I stay aware of my breathing simultaneously?**