Inter-modular connections (IMCs) govern the structural performance, safety, seismic resilience, and reusability of steel modular buildings, yet the existing literature on advanced IMC systems remains fragmented and connection-specific. This paper presents a comparative review of nine research-stage advanced steel IMC systems benchmarked against VectorBloc, the only commercially deployed proprietary connector among the systems considered, selected through a transparent screening of 120 research articles drawn from Scopus, Web of Science, ScienceDirect, and Google Scholar, of which 60 were retained for detailed analysis based on predefined inclusion and exclusion criteria. The ten connections reviewed are the VectorBloc Registration Pin (commercial benchmark), friction rocking, Shape Memory Alloy (SMA) Bolt, Cruciform Plug-in, L-Shaped Bracket with Web Cut-Outs, H-Shaped Bracket, Mid-Depth Self-Locking Plug-in, corrugated web moment frame, post-tensioned rod with friction washer, and Omnidirectional Through-Plate Corner connections. As VectorBloc is a commercially available proprietary product rather than a research-stage connection, it is not directly compared with the other nine systems in this review; it is included solely to provide a qualitative commercial point of reference. Each of the nine research-stage systems are assessed on a like-for-like basis against eight criteria: structural capacity, seismic response, energy dissipation, fire performance, constructability, sustainability, reusability, and cost impact; VectorBloc is discussed against these same criteria qualitatively, as a reference point for current commercial practice, rather than ranked numerically alongside the nine. The comparison shows that SMA bolts and friction rocking connections excel in seismic resilience and self-centring, corrugated web connectors combine high energy dissipation with sustainability benefits, and VectorBloc’s commercial track record offers a useful benchmark for constructability and cost efficiency, although no single system performs best across all criteria. A multi-criteria design framework is proposed to guide future connection development and hybrid strategies, identifying fire performance, long-term durability, and reusability as the most significant research gaps for next-generation modular steel buildings.