I would go with the 8". I just sold my 6" grizzly cause it was too small. You will like it better for longer boards. It sounds like you need a planer more than a jointer though, unless you already have one. Use a planer for the face and a jointer for the edge. I know some people like to run the face of one side on the jointer before running it through the planer, but it really isn't necessary.
If you are gluing panels, then a jointer is a must. Like anything, the nicer the tools the better the results (and faster). Keep checking Craigslist, I found my 8" grizzly almost new for $300 and sold my 6" for $250. Right now Grizzly is selling the 8" for $650 Plus shipping http://www.grizzly.com/outlet/8-Join...HP-Motor/G0586
I would take strong exception to that statement, my friend. The jointer creates one flat surface out of a board with twist or cup by running the surface over the cutterhead then by transferring pressure to the outfeed table, several passes may be necessary to make the entire surface "flat". Then, that flat surface is pressed frimly against the fence to create one perpendicular edge. Now, you can use the "thickness planer" to make the opposing side "flat" and most importantly "parallel" with the original surface.
Your method would jamb a planer in a heartbeat since nothing is flat on the initial pass and may result in a wedged workpiece. However,
I have done what I advise not to do, starting with a "relatively flat"
workpiece. My older planer has rubber feed rollers and is pretty forgiving.
There is no substitute for having both machines and every woodworking professional will have both. My friend's custom door company, where I have worked on occasion, has a 16" jointer, a 20" thickness planer, a 12" table saw, a 42" widebelt thickness sander, and a dual spindle shaper capable of 8" cutters. He has other specific tools for his trade but these are the basic ones.
Myself, I have 2- 6" jointers, and a 13" jointer/planer, 3 thickness planers, and a 24" dual drum sander, dust/generator. I use them daily and could not work without both types of machines. I was also a "shop" instructor at the University of Illinois in the '60's. So, my opinion is based on some real wold experience.