If you were buying a new bike/frame, would post-mount brake be a deal breaker? Beside aesthetic, and maybe harmonic vibration, is there a real difference? The vast majority of new release are going the flat mount route..
No difference whatsoever from a functional perspective. Do consider the availability of post mount calipers - all new models (from Shimano at least) are flat mount (although some MTB models are compatible with road brifters).
