Quote by James R
The difference will be in how bright things look through your scope, and therefore also in what are the dimmest objects you can see.
The light collecting power of a scope depends on the crosssectional area of the tube.
The crosssectional area of a 6inch scope is [itex]36\pi[/itex] square inches.
The crosssectional area of an 8inch scope is [itex]64\pi[/itex] square inches.
So, basically, the 8inch scope has 64/36 = 1.8 times the lightgathering power of the 6inch scope.
Therefore, in terms of the power alone, you'd expect the price of the 8 inch scope to be about 1.8 times the price of the 6 inch scope, which works out at almost $300. However, I'm not sure that the extra cost in materials justifies this increase in price, so I'd advise you to shop around a bit. At first glance, I suspect that of the prices you have quoted, either the 8 inch is too expensive, or the 6 inch is too expensive, for what you're getting.
Personally, I would go for the 8 inch over the 6 inch, just because you'll get almost twice the lightgathering power. You won't regret it. But, it depends on your budget, of course.

Just wondering, but how did you get those numbers for the cross sectional area and why couldn't you just calculate for the available area?
6" diameter: Area = 28.26 in2
8" diameter: Area = 50.24 in2