I don't have a Tang Nano 20K, but I can say a few things. The 20K uses a Gowin FPGA that has more resources and can thus support larger designs. There are many board-level differences also. The two boards have differences in connectors and external pin counts. Pin constraint files for one will not be compatible with the other. I think that either the 9K or the 20K are good for learning about FPGAs. On Amazon, the 9K was cheaper. Hence my choice.
I should also have said. I think this PicoRV32 project would work on the 20K if you change the device type and the pins in the constraint file. To figure out the pins, look at the pinout diagram and the schematic on the Sipeed wiki (Google finds it if you search for Tang Nano 20K).
How much different is the 20k programming from 9k?
I don't have a Tang Nano 20K, but I can say a few things. The 20K uses a Gowin FPGA that has more resources and can thus support larger designs. There are many board-level differences also. The two boards have differences in connectors and external pin counts. Pin constraint files for one will not be compatible with the other. I think that either the 9K or the 20K are good for learning about FPGAs. On Amazon, the 9K was cheaper. Hence my choice.
I should also have said. I think this PicoRV32 project would work on the 20K if you change the device type and the pins in the constraint file. To figure out the pins, look at the pinout diagram and the schematic on the Sipeed wiki (Google finds it if you search for Tang Nano 20K).