For me the 2 TB Fusion Drive is the clear choice. That is because it has now been documented that if you order only SSD, you lose the ability to connect an internal SATA hard drive. Likewise, if you order a spinning drive only, you lose the connector for the internal SSD. The 2 TB Fusion drive is plenty fast for me. In time, I will upgrade both parts of the Fusion Drive on my own when third party options become available. (I am quite comfortable working on the insides of iMacs).
Just wondering where do you see this document?