If the size of the objective is the same, and same prism system, the 8X should be better at low light because its exit pupil is larger (calculated by dividing the size of the objective by the power). There's other considerations, such as that an exit pupil that is larger than the size of your dilated pupil won't do you any good, clarity of glass, etc. but with the same model in different powers the comparison should be straight-forward.
Field of view might be something else you'd want to consider for most hunting applications, and the 8X should also have a wider field of view than a 10X in the same model.
There's lots of online sites that can tell you much more, if you're interested. I don't have any experience with Pentax, do have a Nikon ATB that I like very much that retails for about $280, can be found for about $210-$230. Generally you get what you pay for with glass, so the Pentax very well could be much better. Probably would be wasted on me though, not sensitive enough to appreciate the really high levels of quality, tends to be a bell curve where "improvement" is at a lower benefit/cost ratio in the higher price ranges.
