The calibration of galaxy cluster mass estimators is essential for cluster cosmology, but the different techniques used for measuring masses all suffer from different systematics. This motivates the comparison of multiple techniques across large samples of clusters. I will present the results of a comparison between cluster mass profiles determined from galaxy dynamics using the caustic technique, and from X-ray data assuming hydrostatic equilibrium. We find a large scatter but good average agreement between the techniques. Our results suggest that the so-called hydrostatic bias cannot be large enough to explain the discrepancy between cosmological constraints from cluster number counts and cosmic microwave background anisotropies.