Side-by-side specifications of the Synology DS1517+ and DS1520+, from official datasheets. Differences are highlighted.
| DS1517+ | DS1520+ | |
| CPU | Intel Atom C2538 | Intel Celeron J4125 |
| Cores / threads | 4c/4t | 4-core |
| RAM (base) | 2 GB | 8 GB |
| RAM (max) | 16 GB | 8 GB |
| Drive bays | 5 | 5 |
| Expandable to | 10 | 15 |
| M.2 slots | 2 | 2 |
| Network | 4x1GbE | 4 x 1GbE RJ-45 |
| 10GbE | optional | no |
| Max raw capacity | 108 TB | 80 TB |
| Hot-swap | yes | yes |
| Form factor | desktop | desktop |
| Released | 2017 | 2022 |
Pick the DS1520+ if you want the newer, faster platform: it has a Celeron J4125, can take up to 8 GB RAM, and includes two M.2 NVMe slots for cache. The DS1517+ is the better choice only if you need a fifth bay with direct 10GbE expansion support via its PCIe slot; it uses an Atom C2538 and supports up to 16 GB RAM, but has no built-in M.2 slots.