What makes a big difference in the answers is knowing what type of work you plan to be doing, the woods you'll be using, any space or budget limitations, and if time is a priority or not.
Norm used a 6" DJ-15 for a number of years before he got an 8" DJ-20, but he also notes that he buys the flattest boards that he can so that he doesn't have to face joint. The advise I often see is to get an 8" jointer with spiral cutter head, or even 10" or 12" if you can afford it (or a combo machine), but I find it difficult to justify spending 5x of what I paid for my table saw for a jointer only to have it overwhelm a dust collector costing 10x less than it. Not to mention size and weight issues with some of those monsters. I'm pondering these same questions and haven't answered them for myself so I look forward to the discussion.