This paper compares the performance of two IEEE802.15.4 physical layers in the Smart Building context: 2.4 GHz O-QPSK and sub-GHz OFDM. The former has been in the IEEE802.15.4 standard since 2003, the latter was rolled into its 2015 revision. OFDM promises exceptional performance, in particular in environments with high external interference and multi-path fading. This paper starts with a comprehensive overview of IEEE802.15.4 and IEEE802.15.4g, with a particular focus on OFDM, its design drivers and modes of operation. The second half of this paper presents results from an exhaustive benchmarking campaign of both technologies in a building environment, and discusses lessons learnt. We show how OFDM has a higher range, even at 400 kbps and 800 kbps data rates. We then quantify the importance of frequency repetition in OFDM, and of using a wide communication channel, and we show how the use of OFDM can result in a 2–4 χ decrease in power consumption compared to 2.4 GHz O-QPSK. We conclude by recommending the use of OFDM option 1, with MCS2 for short (<128 B) frames, and MCS3 otherwise.