If you are only going to own one bag, a messenger bag isn't better, to be perfectly honest.
Like Deshi said, I use the messenger bag when I'm not carrying a ton of stuff. I prefer it to my backpack because it fits closer to my body, and I don't have worry about it swinging around and hitting things/people (such as when I'm walking somewhere crowded).
I should also note that I got the small size (Chrome Mini-Metro) for this reason. If I want to carry a lot of stuff, I use a big backpack that I've owned forever.