For hearing devices, it has been shown that external microphones enable to improve the speech enhancement performance compared to only using the head-mounted microphones. In this contribution, we compare the performance of different extended generalized sidelobe canceller (GSC) structures aiming at joint noise and interferer reduction. Recently, an extended GSC structure was proposed in [1], where the external microphone signals are pre-processed by means of the noise-and-interferer references of a local GSC. For the local GSC, the relative transfer function (RTF) vector of the target speaker is assumed to be known, while the external RTFs are estimated blindly. Whereas in [1] a minimum variance distortionless response (MVDR) implementation was used for noise reduction only, in this contribution we propose a minimum power distortionless response (MPDR) implementation instead, enabling to suppress an additional interfering speaker. Simulation results with reverberant recordings show a larger robustness of the MVDR implementation against RTF vector mismatch. However, in adverse conditions the MPDR implementation is found to lead to a better performance in terms of interferer reduction.[1] R. Ali, et al. "Methods of extending a generalized sidelobe canceller with external microphones." IEEE/ACM Transactions on Audio, Speech, and Language Processing 27.9 (2019): 1349-1364.