If you can get picked up by a publisher, it is far more likely than not that the final version of the book will be significantly better than if you'd self published. What's more, the very fact that you got picked up in the first place means that your book is much more likely than not to be at the better end of the scale in any event. And as a general rule, good product sells better than bad product.
Agreed, and just to throw a different perspective into the mix:
When you record a song and are happy with it, you want to share it with everyone you know who likes your songs. It's only when someone you aren't sure of listens to it, possibly in your presence, that you hear its flaws properly and the song begins to sound awful. This, you take into account when you record another song.
When a publisher picks up your work, it's likely that s/he does so in spite of the "flaws". Consequently, s/he is willing and eager to invest his/er
(this is getting/has got silly ) time in making it publication-ready. Perhaps this is also the time the publisher will be making decisions on how quickly you will learn and how much work he will have to do on your next book, so your deal with them will be for a few books, initially.
When self-publishing, every sale is an endorsement of your flaws and you are less likely to learn from them, but equally it's unlikely that more than your admirers will stick with you to the second book, so sales may actually slump. After all, if you're still self publishing then clearly the publishers you sent your opus to weren't sufficiently impressed.
And finally, when a publisher picks up an author, they are already thinking of the cost of publicising your genius with posters, chat shows, movie premier attendances and award ceremonies. The act of meeting you is a percentage of that cost which will only be met after many other cheques have been written. If it looks like you're going to be hard to teach as well, then you might just find yourself with an interesting note in your diary for that day and nothing more.
As usual, Peter explains things much more lucidly than I tend to, but some of the more informative aspects of what I've said above may hopefully hit home for some people.