Low-power wireless applications require different trade off points between latency, reliability ,data rate and power consumption. Given such a set of constraints, which physical layer should I beusing? We study this question in the context of 6TiSCH,a state-of-the-art recently standardized protocol stack developed for harsh industrial applications. Specifically,we augment OpenWSN, the reference 6TiSCHopen-source implementation,to support one of three physical layers from the IEEE802.15.4g standard FSK 868 MHz which offers long range, OFDM 868 MHz which offers high data rate,and O-QPSK 2.4GHz which offers more balanced performance. We run the resulting firmware on the42-mote Open Testbed deployed in an office environment, once for each physical layer. Performance results show that, indeed, no physical layer outperforms the other for all metrics. This article argues for combining the physical layers, rather than choosing one,in a generalized 6TiSCH architecture in which technology-agile radio chips (of which there are now many) are driven by a protocol stack which c hooses the most appropriate physical layer on a frame-by-frame basis.