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 cross-sectional area of the tube.

The cross-sectional area of a 6-inch scope is [itex]36\pi[/itex] square inches.
The cross-sectional area of an 8-inch scope is [itex]64\pi[/itex] square inches.

So, basically, the 8-inch scope has 64/36 = 1.8 times the light-gathering power of the 6-inch 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 light-gathering power. You won't regret it. But, it depends on your budget, of course.