With that said, I am looking at a Shop Fox W1679 & W1745 & Grizzly G0452P & G0604X (which is a parallelagram and not exactly sure what the advantage of that is)
I've seen the 10" JET (JJP-10BTOS), and it is definitely a specialty machine. Its 10" capacity is huge for a jointer, but small for a planer. Also, it is light-weight, with modest power. At the risk of gross generalization and doing the machine injustice, to me it's a solution for people with a very small shop that generally build jewelry boxes, cutting boards, etc. Long boards for cabinetry would be a challenge on this machine.
The Grizzly/ Shop Fox machines you mentioned are solid 6" jointers. I used to have the G0452, and it performed well. As for the difference between dovetailed and paralleogram jointers, see the following from the woodweb archive:
A parallelogram jointer offers four point adjustability for each table. Dovetailed ways units can only be shimmed to account for sag. Also p-tables can be removed and ground and reinstalled if it's warped. D-table jointers must be ground as an entire unit. There really is no real adjustment on d-tables except shimming. P-table has a slightly tighter throat because of the table movement but it's an insignificant reason. P-tables are much more complicated to take off compared to d-tables. My personal favorite table design is the inclines on old Olivers, and Crescents. They are the simplest and best to remove and adjust.
This image from woodworkingtalk might help:
Several people here on the site have the
Ridgid JP0610 and like the machine. At $429 (plus tax), it's very reasonably priced.