There is no blanket right answer.
Each author has to know what she wants, and then decide what she's willing to do to get it and who can best deliver it. Some authors want to make their own cover art. Some authors want more sales. Some authors want to control who can buy their books. Some authors want substantive and copy editing, while others do not want to have to change a word of their prose. No two authors have the same lines in the sand, the same priorities, the exact same goals.
Invariably (with the rare exception like 50 Shades) an established trade press will sell more copies per title than a self-published author. Trade presses invest $ in the books they acquire and pay for editing, cover art, marketing, distribution, etc, while self published authors have to pay for (or do for themselves) all that stuff. On the other hand, trade presses will require you to work with an editor, will decide on cover art and sales price, and will pay you less of a percentage of the sales than you would get with self publishing.
And, of course, good publishers get a thousand times more submissions than they can accept, so authors generally have to go through a lengthy process of submitting, getting rejected, submitting to another publisher, getting rejected, revising the manuscript, submitting, getting rejected, writing a second book, submitting that one, getting rejected..... before they've written something that is of publishable quality and get offered a publishing contract. Self publishers don't have to get anyone's permission. But, if a self publisher isn't able to judge their own book realistically in the context of the marketplace, they're likely to be quite disappointed when they only sell a handful of copies instead of thousands.