The latter is obviously more important, but if your website isn't informative and looks bad who will want to take a risk on your book? Any new writer, especially those that decide to go the Print On Demand (POD) route - via a self-publishing company, understand the frustrations of getting potential readers to take note of their work. You may have a lot more control over the publishing process, but you won't have the marketing power that big publishing firms have. So how can you take advantage of the internet and level the playing field? You have to design something that catches the eye of potential readers.
Start with identifying the top 10 - 15 key words that describe your book. Consider the following: genre, title, audience, character names, themes, publishing company, and author's name.
For example, my book is a Fantasy Fiction novel entitled, Prince Kristian's Honor, Book One: The Erinia Saga. The keywords I selected include: Fantasy, Fiction, New Novel, Tod Langley, Honor, Sacrifice, Demon, Action-Adventure, Wheatmark Publishing, and sorcerer.
I also could have chosen Prince Kristian, Erinia, the dead, etc, but I wanted to focus on the primary themes within the book to attract not only the attention of fantasy fiction enthusiasts but also to highlight that this book has much more to offer. My series of books are a social commentary. So my keywords are meant to attract two separate types of readers with some common desires in a book.
Once you have the keywords identified, you have to design a functional website that relays the information from your book and highlights the words that brought people to your site. *For more information on how your website can come up in a Googlesearch you need to research Google Crawl.
I think the essentials of a writer's website have to be the back cover or even a small synopsis, author bio, media FAQs, release date and hyperlinks to the ordering sites. Consider giving something back to your readers also - a forum or even artwork. The idea is to give something back to potential readers. Most potential readers are going to research the book before they buy it. You have to show them that spending their money on your book is worth it ... that there is much more to expect then what they will get from a book on the shelf at a bookstore. Convince readers that you care about them by giving them your insights, motivations, future plans, and even reviews (and respond to the reviews when you can).
A solid webpage should not overshadow the quality you put into the book. What I mean is ... spending a lot of money for someone to build you an outstanding webpage isn't what's important ... giving people the information they want and need to make a decision about your book is.
Now if I could just figure out CSS and webpage design ... :O)