"This fork is bigger than that one."

Translation: Ten widelec jest większy niż tamten.

[conor.raff]

Am I right in thinking that in these comparatives "niż" and "od" are interchangeable?

The only difference is that "niż" takes acc. case (biernik) and "od" takes gen. case (dopełniak) ?

[immery]

Yes, I think you are right. but:

"od" takes dopełniacz/genitive (kogo? czego?)

"niż" takes mianownik/ nominative (kto?, co?)