It of course depends on the type of work you want to do. I had a little 4" once that I loved, a great little machine, (an old rockwell). Then I got an 8 ". then a different 8". (didn't like the first one). Loved that for about 10 years, but found i had need for a bigger one. Finally i rebuilt an old Crescent 12" (a great machine) and have been super happy ever since. Why such a wide machine? I know lots of folks only use it to "joint" edges. But once you have a GOOD table saw, and use good feather boards or other things, frankly, the edges can be darn clean without jointing. You might joint one edge, to get a straight edge, but after that, you can just cut with the saw. If you start to do more and more work using solid stock, you find that you can buy a lot nicer wood that must be planed and machined to the thickness that you need. Drawer sides and fronts for instance are usually quite a bit different in thickness. That said, when you want to prepare that stock, you don't take the raw wood first to the planer. You often take it to the jointer first, to get a flat, "planer" surface. once you have that one surface, it goes to the thickness planer to make the other side parallel to it. Thus, you can start to see why a wider jointer starts to make a lot of sense. A lot of boards are at least 8" wide. For my 12" Crescent, i fixed up a stock feeder, on the outboard end and that creates all the pressure on the outfeed and pulls the work thru, so GREAT surface. Still, i worked for years on that 8" and it serves well. So yes, i would get an 8".
As for the helical knives? Well, they sound convenient, don't they? But knives aren't that hard to change anyway. And i don't see that they leave a better surface. I know that they talk about "Shearing" the fiber etc, but i think that is just sales talk. Look at the angle that they are to the wood they are shearing. NOT a very large angle. I don't think they shear at all. What i do believe is that they are quieter. There is science to support the fact that there is less "chatter" as the blade hits the wood at so many times per second. There is a blade "hitting" the wood all the time in a helical cutter. but this is just an opinion. By the way, there was an comparison test between jointers of this range a while ago, (months i think) in FWW. Can't remember who won out.
best
paul