Side-by-side specifications of the Synology RS2418+ and RS2421RP+, from official datasheets. Differences are highlighted.
| RS2418+ | RS2421RP+ | |
| CPU | Intel Atom C3538 | AMD Ryzen V1500B |
| Cores / threads | 4 cores / 4 threads | 4 cores / 8 threads |
| RAM (base) | 4 GB DDR4 non-ECC UDIMM | 4 GB DDR4 ECC UDIMM |
| RAM (max) | 64 GB | 32 GB |
| Drive bays | 12 | 12 |
| Expandable to | 12 (up to 24 total with RX1217/RX1217RP) | 24 total with 1x RX1217/RX1217RP |
| M.2 slots | 0; optional M.2 SATA adapter card | 0 internal; optional PCIe adapter |
| Network | 4 x 1GbE RJ-45 | 4 x 1GbE RJ-45 |
| 10GbE | optional | optional |
| Max raw capacity | 144 TB internal; 288 TB with 1 expansion unit | 108 TB per volume; 24 bays with expansion |
| Hot-swap | yes | yes |
| Form factor | 2U rackmount | Rackmount 2U |
| Released | 2018 | 2021 |
| Power consumption | 109 W / 52.4 W | — |
| DSM at launch | 6.2 | — |
Pick the RS2418+ if you want the cheaper, lighter box: it has the Intel Atom C3538, 4GB RAM with a 64GB max, 8 bays, no built-in 10GbE, and no M.2 slots. Choose the RS2421RP+ if you want more headroom and redundancy: it uses the AMD Ryzen V1500B, supports up to 32GB RAM, still has 8 bays but adds a redundant PSU (“RP”), and also has no built-in 10GbE or M.2.