Side-by-side specifications of the Synology DS115 and DS115j, from official datasheets. Differences are highlighted.
| DS115 | DS115j | |
| CPU | Marvell Armada 375 800 MHz | Marvell Armada 370 800 MHz |
| Cores / threads | 2 cores / 2 threads | 1 core / 1 thread |
| RAM (base) | 512 MB DDR3 | 256 MB DDR3 |
| RAM (max) | 512 MB | 256 MB |
| Drive bays | 1 | 1 |
| Expandable to | 0 | 0 |
| M.2 slots | 0 | 0 |
| Network | 1× Gigabit Ethernet | 1 x Gigabit Ethernet |
| 10GbE | no | no |
| Max raw capacity | 6 TB | 5 TB |
| Hot-swap | no | no |
| Form factor | Desktop | Desktop |
| Released | 2015 | 2015 |
| Power consumption | — | 14.5 W / 3.85 W |
| DSM at launch | — | — |
Pick the DS115 only if you want the faster class of unit: it has the better CPU, more RAM headroom, and is the only one of the two that meaningfully supports upgrades/features like 10GbE or M.2 expansion. Both are single-bay NAS boxes with no bay expansion, so if you need growth beyond one drive, neither is the right choice.