I personally prefer reading silently for both MiiNiPaa's reason and because I would find it incredibly awkward to be narrating a book to anyone in the vicinity and I would be very uncomfortable sitting in a room with someone reading a book out loud for any length of time.
@Bogeyman, I often employ a similar technique where if I am planning something or trying to work something out I find it much easier if I can walk around and talk to myself (out loud) about it. But I would feel like a colossal idiot doing this in the presence of people.
There is no difference between reading out loud and reading silently for me, because when I read silently I hear the voice in my head as I read. It limits my reading speed for sure, but it boosts my comprehension.
I'm involved with a school for children with language-based learning disabilities. Language is deeply related to thinking and learning. Some people learn well from hearing. Some people learn from reading. Some people learn from doing.
Some people retain written material if they read out loud because they can hear the words. If this works well for you, then do it.