I voted 'depends on the price' because generally the contents of the book is the most important, but hardcover books are good because it lasts longer, especially useful in the hands of a spack like me! However, I wouldn't say I 'refer' it to soft cover books, because soft cover books are generally cheaper and portable, great for trashy novels I wouldn't even normally admit to reading
Hardcover is best, but I'll take a cheap softcover for novels (more portable, etc). For my old Victorian engravers, though, like Gustave Doré? Hardcover, and no later than 1930 or so. (the reproduction in modern works, frankly, sucks)
On a side note, I remember this one book that I ended up buying because I couldn't get over how good it felt to hold. It was incredibly soft and flexible, but the spine wasn't broken or anything. While I was reading it, I kept getting distracted by the flexibility of the book.
Oh, and I prefer hard covers that have an illustration on the actual cover, so I can take the jacket off (if it has one) and still enjoy the cover art.
If I was buying something to keep I might go for hardcover, but then I take really good care of my books, so I can have soft cover and be just fine.