DS420+ alone vs DS420+ with DX517 expansion: 4 bays vs 9 bays for growing storage needs

The DS420+ alone gives you 4 bays, but adding a DX517 via eSATA jumps it to 9 bays. This is the key difference: the expansion unit is essential if you outgrow the base unit, but it adds cost and requires an eSATA connection.