In my mind, it really depends upon the book. Non-fiction and reference works are worth more in my mind, both in terms of utility and long-term use. So I'll buy the format that I think will be more useful as long as the prices are within 20% or so. If the difference is more than 20%, I'll go for the cheaper one.
Fiction is different though. I value some fiction, but most of it is purely entertainment in my mind. This means that I'm virtually all of my purchases are second hand copies. Since there are no second-hand ebooks, the prices virtually always drive me to pbooks.