I've used both. The quality of the glass is more important than the magnification. I don't agree with the common assertion that you need 10xs in the open west. I live in western Wyoming and find my 8.5x swaros more than adequate for anything I wouldn't need to put a spotting scope on. Side by side with the same model in 10xs I found that the difference in finer details was negligible, but the wider field of view of the 8.5xs to be a real plus.
There is a personal preference involved, so I wouldn't say one is better than the other. You would do well to look through them side by side, in what ever models you decide on. If you have poor eyes in terms of contrast and clarity, you may find more magnification is better.