Galdor: I'm Polish and this being game made by Poles and based on a Polish novel, it is kind of obvious what I would prefer :)
However, I believe that the quality of the English VO is way above the Polish one.
Putting the actors aside, the technical side of the VO in English beats the "original" ones, they must have been recorded in a better studio or, if it was in the same one, on a preferable conjunction of spheres or something. The voices fit the environment and don't sound like they've been recorded in a big warehouse.
As for the actors, it is all relative and up to everyone's taste but
for me almost everyone sounds better in English, with Geralt being an exception <- He's good in Polish VO's.
So yeah... I've downloaded the DLC, played part of the prologue in Polish and couldn't stand the contrast in quality. Finished the game in English twice already.
"Ever heard of doing it Witcher and Striga style?" :D
I agree. Even though Polish is my native language I quickly switched to English VO and Polish subtitles. I find English VO more natural, betted played and just more "fitting".