Multiple-input multiple-output (MIMO) and carrier aggregation (CA) are two key MAC/PHY layer technologies employed by both user equipment and base stations to boost data throughput in 5G networks; yet, their respective contributions on real-world throughput improvements remain largely unexplored. Although both approaches conceptually rely on parallel transmissions, they differ fundamentally in their implementations: MIMO exploits spatial diversity through multiple data streams within a band, while CA aggregates spectrum across multiple bands. In this work, we present the first comparative study of MIMO and CA throughput gains in operational 5G networks. Using extensive measurements with commercial smartphones over all three major US cellular operators during a cross-country trip (from LA to Boston, 5700km+), we first present the current state of deployment of both technologies in today’s 5G networks. We then disentangle their combined effects on throughput, quantifying the relative contribution of each technology to overall performance. Finally, we analyze how throughput scales across higher-order configurations of each technology, considering different MIMO transmission ranks and numbers of aggregated carriers, providing insights for future 5G deployments.