If you want fast data reads and writes, the SSD is the way to go. If you want massive storage space, the traditional HDD is the ticket and Seagate has new offerings. Seagate now has 16TB HDDs in its Ironwolf, Ironwolf Pro, and Exos lines.
The Exos X16 is helium-filled and sells for $629 in SATA and SAS versions. The IronWolf 16TB drive is SATA only and sells for $609. The IronWolf Pro 16TB is also SATA only and sells for $664.
One difference in the IronWolf and IronWolf Pro is a warranty; the standard IronWolf has a 3-year warranty while the Pro version and X16 have 5-year warranties. The Pro version also has two years of free data recovery reports PCWorld.
IronWolf Pro has 250MBps data transfers with the standard IronWolf at 210. All three drives are 7,200 rpm units with 256GB of DRAM cache. The X16 also supports RSA 2048 firmware verification. The X16 HDD is shipping now with the other two drives shipping soon.