Which blu-ray?

Catch 22 basically...There are three major versions of the film: US Theatrical release, European theatrical release, and the Extended UK Blu-ray release.

Comparison of the US and European theatrical (labelled extended):

http://movie-censorship.com/report.php?ID=121

Comparison of European theatrical (aka. US blu) with UK blu:

http://www.movie-censorship.com/report.php?ID=357350

As you can see, there is a case to be made that either the US blu or UK features more - I prefer the scene of Conan and Subotai talking in the UK one, and I like that the Princess helps him get back into Thulsa Doom's Mountain of Power. So that's the BIG difference in the films, as for the discs:

The US disc is the full extended version, with all animal violence (including horse falls) intact, but the audio track has some errors - the chorus in Basil Poledouris' score was accidentally not included on a couple of places which hurts the film.

The UK disc has a slightly different cut, as I pointed out, but the horse falls in the Battle on the Mounds are cut, and the music is affected. Obviously, this is a big no no as it hurts the impact and flow of the great score.

NOW it used to be the case that the UK blu-ray could be cheated - if I recall correctly, picking another language (French I believe) on the menu, meant you would enter the menu in French and could then watch the full, uncensored version of the film (and swap back to the English language track with your remote). So this is the best disc, right? Longest cut, uncensored, music intact? Well, I believe the version available now DOES NOT let you do this. Look at reviews on Amazon for more details.

You could risk it, or you could buy a copy from amazon.fr of their disc, which I believe is just as good.