Side-by-side specifications of the Synology RS3618xs and RS3621xs+, from official datasheets. Differences are highlighted.
| RS3618xs | RS3621xs+ | |
| CPU | Intel Xeon D-1521 | Intel Xeon D-1541 |
| Cores / threads | 4 cores / 8 threads | 8 cores / 16 threads |
| RAM (base) | 8 GB DDR4 ECC UDIMM | 8 GB DDR4 ECC UDIMM |
| RAM (max) | 64 GB | 64 GB |
| Drive bays | 12 | 12 |
| Expandable to | 36 total with 2 expansion units | 36 |
| M.2 slots | 0 | 0 (via PCIe adapter cards) |
| Network | 4 × 1GbE RJ-45 | 4×1GbE RJ-45, 2×10GbE RJ-45 |
| 10GbE | optional | yes |
| Max raw capacity | 216 TB | 1 PB single volume |
| Hot-swap | yes | yes |
| Form factor | 2U rackmount | 2U rackmount |
| Released | 2018 | 2021 |
| Power consumption | 142.76 W / 55.05 W | — |
| DSM at launch | — | — |
Pick the RS3621XS+ if you want the newer/faster CPU, higher RAM ceiling, and built-in 10GbE; it’s the better default for a home/SMB buyer. The RS3618xs is older and generally less capable, though both are 12-bay rack units with expansion support and neither has built-in M.2 slots. If you need the extra headroom and longer-term relevance, choose the RS3621XS+.