The UK edition is being converted to Region 1 and sold as a Best Buy exclusive from Tuesday (July 26) to September 20. Then the official US version is being released on the 20th to all retailers and the UK version is being discontinued.
Here are details on each.
The UK version is a 4-disc set with the 13 episodes and deleted scenes. If it is a direct port of the Region 2 set with no changes, it won't even have chapter stops. It has the UK opening theme song, the one that is now used in Season 2 in the US.
The US version is a 5-disc set with the miniseries, 13 episodes, deleted scenes, commentaries, the "Lowdown" and a Behind the Scenes featurette. This version renders the miniseries DVD pointless because ALL content on the miniseries disc is in this set. It has the US opening theme song that is no longer used as of season 2 (though at the end of Valley of Darkness, 2x02, it is used in the show itself, just not the intro).
I'm giving my miniseries DVD to my uncle and getting the US version - I like the US theme better and I want chapter stops, damn it! (And the commentaries.) Maybe I'll get him the UK version for Christmas or something, he doesn't care about extras for the most part.