Maybe you have a second printing of Prince.
Anyway, as has been mentioned, Empires has fixed the bendiness and also the card backs, it's just the card stock is slightly different in texture from pre-Adventures. Adventures and Empires have different card stock it seems, but this really only makes a difference for Empires if you don't sleeve.
I believe all copies of Prince were printed at Altenburger (though someday that won't be true).
The card stock should be identical between Empires and Altenburger-printed sets. The reason is, Cartamundi bought ELM, and we said, we want the identical card stock, surely that's possible at this point, and they said, yes it is and okay. So, it would be a big surprise if it wasn't identical.
I remember when that guy said his Intrigue cards were thinner, and we measured thickness and they were not, but did not realize he was reporting a symptom not the disease, that they were not thinner but were different (in particular bendier). So maybe "these cards are a different card stock" is a symptom of something else, some actual difference, dunno. It's not a symptom of them being a different card stock though.