I usually use 8x32, when I'm carrying camera gear. If only going with binos, then 8.5x42 swaros. If I know it's long distances (e.g. Alaska, ocean/beach, fields), then I might take 10x42. 10x42 is what I usually leave in my truck.
At shorter distances, I do not notice a significant difference between 8.5x42 and 10x42 in terms of magnification, but I'm not counting feathers either. At longer distances, I notice the difference more. What I do notice at shorter distances is that I need to fiddle with the focus more on the 10s to keep a crisp view, the 8's are more forgiving. 7's are even better in that regard.