Contact and non-contact (optical) vibration detection methods, used in a resonant ultrasound spectroscopy application, are discussed in a comparative manner. The noise-floor for three different methods of vibration detection are quantitatively compared using a unique experimental configuration which employs spherical resonators. Spherical resonators, when resonating at their “breathing mode,” have the special characteristic of vibrating uniformly in all directions with the same displacement. Noise-floor comparisons are made utilizing this unique mode of vibration to simultaneously measure vibration displacements for all three methods without changing the mechanical input coupling to the resonator. Among other things, in this application, we demonstrate that contact vibration detection systems do not necessarily perform better than optical methods in terms of noise-floor and cross-talk considerations.