This is all very subjective, at least for me. It's kinda like asking me if I prefer wall art to have a landscape or portrait orientation.

I used to really dislike mass market paperbacks, but certain books seem to really work with that format. That dislike probably came from seeing so mass markets with terrible covers while growing up. If I really enjoy a book or it's something I'm almost certain to enjoy, I prefer to get the hardcover assuming it has the same cover as the paperback. For my favorite books, I have multiple editions and I typically get the newer ones first, then seek out the original covers later just to have them.