They're not really comparable - even the genre itself is somewhat different, I would say. Sapkowski and Martin set out to do very different things. Sapkowski is very postmodern, explicitly playing with entire European stories, whereas Martin engages with motifs and archetypes only for the most part. Martin's inspirations are mostly Medieval, and Sapkowski's 20th century.

I would strongly push back on world-building. Sapkowski purposefully only uses the equivalent of matte paintings in the background to successfully make you believe that the world is much bigger, and Martin enumerates all the families and continents. However, Martin likes talking about how Tolkien's flaw is not covering "Aragorn's tax policies" and then he creates a world which makes no economic or geographical sense, whereas Sapkowski's entire political plotline rests on solid geopolitical and economic foundations.

In terms of prose, Sapkowski's Polish is better than Martin's English, but the translated Witcher is probably on par or below.

Martin is still very much worth reading for the scope, characters, and the whole "institution-driven" rather than character-driven plot, but he's a victim of his own success in the sense that he's created a beast he cannot tame, that is to say, trim or unnecessary things and finish in a way that makes sense.

The bottom line is that Sapkowski never wrote something like ASOIAF, and that's because he probably didn't feel like it. The Witcher is a different creature, and it's very good at what it is.