Assuming all [other factors] are equal … you probably buy on weight, because you subconsciously anticipate a longer reading experience and, all things considered, good experiences that last longer are better than short ones. Remember that the actual cost of the paper and ink is only a small component of the retail price of a book — around 10-15%. Increasing a book block’s size from 150 pages to 180 pages is cheap. And so, from the 1960s to the 1990s, publishers unconsciously trained readers to expect longer novels.
This, of course, begs the question: what is the ideal length for a digital novel? I suspect that lengths would vary according to type of digital medium: short for the web, 300-500 pages for pdf, and possibly infinity for iBook/Kindle books. Hold that thought; I think it’s worth coming back to in the near future.