This is almost as controversial a topic as single versus dual cam, carbon versus aluminum, etc...
I prefer 8x for hunting and bowhunting in particular but you have to keep in mind where I hunt and the conditions I hunt in. In my neck of the woods most shots are relatively close due to foliage, topography, etc... If I can see more than 30 or 40 yards in any direction then I am not in a thick enough spot. I may have some areas that I can see great distances but I am not set up there because of it.
An 8x binocular (in comparison to a 10x) provides a brighter sight picture, larger exit pupil and greater field of view and greater depth of field.All can be especially beneficial at close to moderate ranges in low light situations. However, you mentioned glassing way out to 400 yards. In which case, the 10x should hold some advantage , assuming you can hold the binocular steady enough to actually gain something from the increase in magnification. I say this because it is a generally accepted fact that 10x bins are more difficult to hold as steady an image as 8x bins. I have personally found that higher quality glass and better optical coatings actually are more beneficial to seeing detail at longer distances in comparsion to just increasing magnification in a less expensive binocular.
Just some food for thought.