Your question is a largely personal one. Magnification preference varies as much as people do. For some sort of intelligent response, we really need to know what kind of hunting. I agree with Bitteroot in that a narrow fov is a problem with some 8.5x models.
Stated from my purely personal perspective, IF and only IF I was in need of ONE binocular for hunting, of your two choices, I'd hit on the 8.5x, unless it is one of the very narrow (too narrow in my view) Minox. I say Minox because they were the first name to pop to mind in having both 8.5 and 10x in 43mm configuration. In which case I'd do the 10x which has the same actual fov, but at 10x will have a much larger apparent fov. I have and use in the following order 7x most often, 8x next and 10x least. Each has its place.
Keep in mind that as magnification increases the steadiness of the user comes significantly into play. Movements in your arms and hands are magnified by the magnification of the binocular, so the reason many prefer 8x is simply that they are easier to hold steady. A steady 8x image will show you more that a shaky 10x will. Another thing to consider is that magnification increases the effects of heat mirage as well. I have been in conditions where I could actually use my 7x where the 10x was toast. I'm primarily a high desert Mule deer hunter and personally see no particular disadvantage in 7x. Sure, sometimes 10x can't be beat. I have a 8.5x44 Swift Audubon porro binocular and think the 8.5x is a very good choice for a one glass user. I don't know what the budget is in your case, but Swift also makes a very nice 8.5x44 Audubon model roof prism too.
On the other hand there are lots of people who can glass steady enough with a 10x that they prefer the larger image size presented.
Where you are is really something that is your call. However a lower magnification glass will have a wider fov, will be a little brighter, since it has less magnification, there will be a little less eyestrain. When comparing the same models of different magnifications from the same manufacturer it seems to me the resolution per each 1x of magnification is a little better in the lesser magnification of the two.
Edited by Klamath - October/13/2010 at 11:50