Depends what you are doing. Woodland mostly then 8X, hill mostly then 10X. Neither makes that much difference, but if you have a variable power scope I set it same as my binoculars (I own both mags, cheap Minox in 10X and Zeiss in 8X).
If one or the other is available and a good price just buy it and stop worrying. Do note that at 10X mag they often fit larger objective lenses so the binoculars themselves are heavier.