If you have a 6" and want an 8", you have realized that a jointer is not a machine merely for "jointing" or straigtening one edge. If that is all it was for, then you woudl probably rarely ever need anything larger than 4". You have realized that it is very necessary to face joint lumber, before being put in the planer or (thickness sander if you happen to have such a machine). I have an 8" Delta, and have loved it. (had a Grizzly and didn't, so sold it after a couple of years, but maybe they are better now) At times i wished for wider still, but lived without that, running the wide stuff thru a thickness sander instead. Finally last year I rebuilt an old 12" machine and absolutely love it. I still use the 8" quite a bit, but the 12" is wonderful. I don't understand the last question perhaps, because if you can afford the space, and found a 12" (four knife) machine for comparable price to an 8" machine, there is no question. Yes, the 12" machine takes up more space, requires more power, but invariably it will be a much better machine. As to spiral cutters? It is nice to not have to take your blades to be resharpened, i admit, but if you are careful, they don't need to be resharpened any where near as often as saw blades do. And i sincerely doubt that they cut any better if as well, as straight blades. The "shear" action is probably more marketing than anything else. The only other good thing about them is that they are probably quieter. That is the reason that some industrial shops have gone to conical heads. Different beasts. But, just an opinion.
best
paul
