The main differences between the Dell S2721HNM and Dell S2721HN monitors are their panel types, design aesthetics, and additional features. The Dell S2721HNM typically features an IPS (In-Plane Switching) panel, providing better color accuracy and wider viewing angles. On the other hand, the Dell S2721HN may come with a VA (Vertical Alignment) panel, offering better contrast and deeper blacks. They might also have differences in design, bezel size, and stand features.