Given this is such a new system why don't you buy the system with the larger SSD already installed (PCIe vs SATA)? 3.1 GHz 21.5" iMac with 512 GB flash.

Then if you want to add in a better hard drive or a second SSD. Thats what I would do.

That way you get the benefits of ultra fast M.2 PCIe interface of the SSD blade drive which is now using NVM Express and then you can use the slower SATA III (6.0 Gb/s) for your second drive.