Here's what I know about the new SSD in Mac Pro so far:
Controller: Samsung (PCIe based, not available yet so not sure if it's SATA or NVMe)
NAND: 21nm Samsung MLC
Capacity: Apple's demo unit is 256GB but with current NAND you can fit up to 1024GB (1TB) of NAND on the PCB
Connector: Proprietary (i.e. not M.2)
Controller: Samsung (PCIe based, not available yet so not sure if it's SATA or NVMe)
NAND: 21nm Samsung MLC
Capacity: Apple's demo unit is 256GB but with current NAND you can fit up to 1024GB (1TB) of NAND on the PCB
Connector: Proprietary (i.e. not M.2)