Mobile contact tracing apps are -in principle -a perfect aid to condemn the human-to-human spread of an infectious disease such as COVID-19 due to the wide use of smartphones worldwide. Yet, the unknown accuracy of contact estimation by wireless technologies hinders the broad use. We address this challenge by conducting a measurement study with a custom testbed to show the benefits and limitations of Bluetooth Low Energy (BLE) in comparison to distance estimation by ultra-wideband (UWB). Our results confirm that BLE-based distance estimation is not sufficient in real scenarios where smartphones are shielded heavily by the users' bodies. Yet, multi-path signal propagation reduces the effect of body shielding. Finally, we demonstrate that UWB is more robust to the environment than BLE.