Like I want to compare what things look like under the Orion SkyQuest XT10 and the Orion SkyQuest XT8 telescopes
How big of a difference would it be?
It is pretty tough to describe, since it depends on a lot of related issues like light pollution, eyepieces, and your own observing skills. As Tina suggested, the only way to really compare these is to try them yourself. We can tell you that the 10 inch has roughly 36% more light gathering capability than the 8 inch, but to translate that into a level of brightness as perceived by your own eye is an exercise in futility. I personally own an 11 inch SCT with premium eyepieces, and under clear skies I can see more than most people can imagine. But some folks can look through my scope at a target that is absolutely stunning to me and see virtually nothing. They don’t quite know how to look; it takes some practice to see what is out there.
So, if I were you, I would check out the views at a local astronomy club star party. If that is not possible, I would go with the 10 inch.