Although these products have the same DSP IO count, they are indeed very different.
- nanoDIGI is all digital (i.e. SPDIF/TOSLINK in/out). It doesn't have any analog in/out.
- 2x8 is all analog but can be made to support digital input when adding a DIGI-FP card.
- The 4x10 HD is essentially a 2x8 with a DIGI-FP card. So the same applies.

If you want to use your own DACs and only require digital input, use the nanoDIGI.
If you want the DACs onboard and may require analog input, use the 2x8 or 4x10 Hd.