Revenue - Commercial publishers will pay you a 6-10% royalty on the sale of each book. If you self-publish, you pay the publishing costs, but keep all of your revenue. So what do you want? All of the revenue or 6-10%?
Control - (I know we have some control freaks out there) Commercial publishers will control your content (have you rewrite it to suit them), your cover design and even your book title. If you self-publish, you control it all. You keep your voice.
Ownership - This is something a lot of writers don't understand. Commercial publishers own the books they publish and pay royalties. If you self-publish your book, you are truly the owner.
Selection process - (A major reason that people self-publish) If you're not writing on a hot topic, in other words if you're not the flavor of the month, a commercial publisher will not publish you. That's why a lot of good writers end up keeping their manuscripts in their basements.
Vanity Presses - A vanity press is a publisher who will print anything and thus is not respected in the industry. They derive their profits from charging the author higher than normal fees for editing, printing, cover design and layout. You can write "I love myself" on 100 pages and a vanity press will print it. If you're going to pay for services, you're better off self-publishing.
Now you've got some facts to consider. Give these points careful thought and Happy Publishing!