I will try to answer in general for the differences in FRAM and nvSRAM in brief.
nvSRAM will read/write at faster speeds than FRAM as they are optimized for this and they will require an additional Capacitor for Autostore functionality. If speed of operation is a design requirement you can opt for nvSRAM. nvSRAM uses charge to store data internally.
FRAM is optimized for low current consumption which leads to more battery life (one of the critical design parameters for portable devices) and does not need any additional components. FRAM uses crystal polarization to store data internally. Let me know if you want more detailed analysis for the two.
nvSRAM architecture has two cells to store the information. One is the volatile cell just like a simple SRAM will have. Hence you can read and write to this cell without any limits just as the case with the SRAM cell. It is due to this cell the read/write speeds for nvSRAM is comparable to that of a SRAM. The other cell is the non volatile cell, which gives the memory its non volatile feature. The user can shift the data from the volatile cell to non volatile cell as per their requirements using the store functionality and for this data is written to the non volatile cell. Since the number of writes to a non volatile cell is limited (true for any non volatile memory) hence the datasheet mentions a limit of 1 million writes to the non volatile cell. For most applications this limit is suffice. )