I used to have 4 bay DAS and docking station. junk DAS cause it RIP just after 2 years; docking just collecting dust--rarely used and port really do collect dust.
The issue with DAS is mainly compatibility. with eSATA connection, many Motherboard eSATA ports do not support port multiplier and it only see one drive; with USB3, the transfer speed is lower than eSATA and also depends the USB3 controller. whats more, Storage diagnostic utilities may not be able to see all drives even though the OS can. The advantage of DAS is when it works, there's nothing you need to do, HDD behave just like internal drives. no danger of shorting or knocking the drives like docking station.
I ditch docking station year ago, the slots will wear out over time and because they are mostly facing up, dust will accumulate or worst, liquid may accidentally drop in to cause short. for a simple task they do--holding one or two drives, they took up multiple times of the drives they hold. They have the same problem of DAS above, except for single drive. but if you only need one drive, a USB3 to SATA adapter can do the job just fine, very portable at less than the size of iphone3.
Now I only use ext HDD to store backup, because its stone alone, no compatibility issue, only the limitation of USB speed. cons is multiple drives need multiple power adapter. for some model, the inside is simply a normal SATA HDD, if you want, you can ply open the case and use as int HDD, after warranty over.
Like this:
http://www.amazon.com/Seagate-Expansion-Desktop-External-STEB5000100/dp/B00TKFEEBW/
Its actually
cheaper than int HDD of similar size. Local 2x 4TB --$199 for seagate/$186 for Toshiba--+ 2 bay dock at $70 =
$468/$440, Amazon Seagate ext 5TB US$134@1.35:1 = ~$181, 2x = ~
$362. Save $78~$106 but get 2TB for free, its a no brainer for me. unlike Toshiba and WD HDD from Amazon, Seagate HDD warranty is international. in case of HDD failure, few dollar postage to Changi and get a replacement back in 2 weeks--deliver to your doorstep.
BTW, the above seagate is only 1 year, this
http://www.amazon.com/gp/product/B00R45V3SW/ is 2 years. 2x 5TB is ~$405, still few tens cheaper than 2x 4TB + dock locally.