Guess it depends just what it's going to be used for. Most 2 bay units are junk (IMO) with slow read/write speeds. Even some of the 4 bay units like the Netgear NV+ have transfer speeds under 15MBs.
You might look at the Qnap units. We just recently purchased two used units, one is a 6 bay and the otheer an 8 bay, both came filled with 2TB hard drives.
We set one up as Raid 5 and the other as Raid 6 with transfer speeds at over 80MBs. Picked up the 6 bay for $400 and the 8 bay for $600
We now have 2 - 4 bay NV+'s sitting on the shelf - only good for home media storage these days but it takes forever to copy files to it compared to a larger Qnap unit. Thought about using them for backups but it would take like forever to 'restore' a client's files.