Hi Astroboyz welcome to IceIn Space
Well, if light gathering is dependant on area of the objective, then the difference is significant
6 inch = 28 sq inch surface area
8 inch = 50 sq inch surface area.
(however, the respective size of the central obstruction (secondary mirror) would also affect this, slightly.
My first scope was a 6" Newt and my second scope was an 8" Newt.
Practically, the 8" produced a slightly brighter image of DSO's but you had to look close to see any greater detail. (which there was but not hugely apparent from casual observation).
What was noticeable was that the 8" was much more awkward to handle and move about. So if this is a factor, it should be weighed into the decision.
But the two general rules of thumb are
1. Go for the biggest aperture you can handle (and afford), and
2. The best telescope for you is the one you are going to get out with and set up, and use more often.
A further disclaimer: For observing a Dobsonian mount is easier to set up and use than an Equatorial mount. But Equatorial is virtually essential if you are going into astrophotography. Other than that they are both Newtonian scopes, except that on a DOB mount you can have a collapsible scope, that makes handling much easier.
Sorry if this is too much information, but it all is relevant to a choice