Flat base bullets generally have an edge, especially in barrels that are less than perfect. That is, tight spots and loose spots. Realistically, I seriously doubt you'd be able to tell any difference in the inherent accuracy between these two styles in the vast majority of rifles out there. What you're more likely to find, is that your rifle has a preference for one or the other. If that turns out to be the case, that's the one to use in that rifle.