Side-by-side specifications of the Synology DS1515+ and DS1517, from official datasheets. Differences are highlighted.
| DS1515+ | DS1517 | |
| CPU | Intel Atom C2538 | Annapurna Labs AL-314 |
| Cores / threads | 4/4 | 4 |
| RAM (base) | 2 GB | 2 GB |
| RAM (max) | 6 GB | 16 GB |
| Drive bays | 5 | 5 |
| Expandable to | 15 | 15 |
| M.2 slots | 0 | 2 |
| Network | 4x GbE | 4x1GbE |
| 10GbE | no | optional |
| Max raw capacity | 30 TB | 75 TB |
| Hot-swap | yes | yes |
| Form factor | desktop | desktop |
| Released | 2015 | 2016 |
Choose the DS1517 if you want the cleaner long-term buy: it has a faster Intel CPU, supports more RAM, and is the one with native M.2 SSD slots for cache. It also retains 5 bays and can expand with a DX517, but unlike the DS1515+ it has no built-in 10GbE option. If you need 10GbE specifically, the DS1515+ is the more attractive choice.