Why is focusing on the breath more difficult than focusing on Mantras, and how important is it before vipassana?

I have been doing mantra meditation for years and saying "Aum" for a few minutes makes me go deeper in meditation within like 10 minutes. But focus on the breath is difficult compared to the mantras. What is the difference between the results of breath vs vipassana? I can hardly do 10 conscious breaths (feeling peace). Any suggestions for betterment? On the other hand doing vipassana makes me feel very strange (like perceiving vibrations) in my entire body. In mantra meditation, it feels like something (kind of waves) is coming out of my body. Am I on the right path or is it hallucination?