The idea is to attune your mind and inner ear to the vibration of the universe. Both are symbolic of it and remind us of that humming sound that is within and all around us. Both help us to become more in touch with the inner sound and light of God. My suggestion is to only use the A U M as practiced on the download for now. In most cases it will take you in deeper than chanting OM but not too deep unless you are ready. In the beginning on the highest form of meditation I prefer the three syllable way of chanting in your mind better than the one syllable you describe. Also because I have taught so many in this method if you have any questions it is much easier to answer and help you if you are practicing in the way that I have taught, rather than try to explain or correct something that comes up with another teaching. Usually it is best to only use one way at a time. This also helps with you tracking your progress. If you are trying several or mixing several ways, when you have results you may not know which way achieved which results. Your results and miracles may not always come right when you are meditating, but come on account of the meditation you have been doing. If you have been practicing a different way, and do not want to stop for a while, I suggest that you wait to practice this type until you can at least try it by itself for a while. When you do, see which one works best for you.
When we are choosing methods and a path, too many choices can be a stumbling block. I have seen many become very confused trying to do more than one at a time. Even if both paths say the same thing in different ways it can be quite confusing. I say find the one that seems to agree with you most, and use it alone. When you have gone as far as you can with it investigate others, building on what you have learned. When you find the one that agrees with you most, where you are at this point, put all you have into this way. Most important give it a little time and see how it works for you. Eventually you will find the way that is for you completely and high enough to take you all the way.