Woodworkers state (over and over) that an 8″ jointer trumps a 6″ jointer every time. Further, that if you buy a 6″ you’ll only end up getting an 8″ within the next couple years anyways.

Heck. Before I offer my five reasons to get a 6″ I’ll even run through some of the most common annecdotes on why to get an 8″:

“I found my 6 inch jointer was almost useless. Top quality hardwood must finish to 6 inches wide to earn the FAS grade, which means that it must start at least 6 1/2 inches wide, which means even the narrowest boards won’t fit on a 6 inch jointer. Since wide boards bring a premium, I think you will find most boards are between 6 and 8 inches wide.”
Charlie Plesums

“I bought the Grizzly 6″ jointer, 7 months ago and within the first month started kicking myself for not spending the extra money for the 8″ one. If you are into getting rough cut lumber from a sawmill, air drying it and then flattening and planing the boards–which is what I do for my stock– 8″ is the minimum. Right now, I’m looking to upgrade to a 10″ jointer.”
Chet Parks

“I sold my Delta 4 years ago and bought a used General 480 8″ machine. Wow…. the longer bed is the most important feature to me, then the HP then thirdly the width.”
Jim Shaver

“Everyone who buys a 6 and has space for an 8 wishes they had the 8.”

Etc, etc, etc… Read a bunch more in the forum links below.

Now here are 5 reasons to get a 6″ instead of an 8″:
1) You have the cash now for a 6″ and won’t have the cash for an 8″ anytime soon… and you’re rip-rarin to start working on projects.

2) You want to get a jointer and a planer at the same time and have limited cash for the foreseeable future.

3) You have limited space and simply can’t fit the 6″ jointer in your workshop.

4) Your projects don’t include overly wide or long boards (often 8″ Jointers are recommended because of longer beds).

5) You read J Grout’s guide to Jointing 11″ wide boards on a 6″ jointer: Jointing wide boards

“When I first started flattening stock on a jointer, I found that many times the material was wider than the jointer I had. I went through the “cut the stock down and re-glue” phase, then a friend and mentor showed me how to accomplish this task without cutting up wide stock. Basically the job is pretty straight forward. It involves the use of an auxiliary guard, which replaces the factory-supplied hinged guard, a set of winding sticks, and a good eye.”

If none of those 5 fit you then, well, 8 inch or more it is!

Jointer Resources from ToolCrib:
7 Table Saw Jointing Jig Plans: Straight Edge, No Jointer

6 Inch vs. 8 Inch Forum Debate Resources
Jointers 6″vs8″ help!
6″ ridgidjointer and a ridgid planer vrs a 8″ grizzly jointer
General 6″ Jointer or BusyBee 8″?
8 inch vs. 6 inch jointer
Jointers – 6″ or 8″?
6 vs. 8 jointer
More on Jointers
Please help me choose a jointer!
Sorry! Another jointer question
6 or 8 Inch jointer
Joiner 6″ or 8″