In addition to each set having all 13 episodes of Book 4 Balance, they will both include some bonus features.
The Blu Ray will have episode commentaries for all 13 episodes, while the DVD will have them for just 4 episodes (I assume K401, K402, K412 and K413). The Blu Ray also includes a poster by Bryan Konietzko plus the following features: Kuvira vs. Prince Wu, Republic City Hustle Part 1, 2, and 3, a New York Comic-Con Featurette and “The Making of a Legend: The Untold Story – Part II.”.
Right now we don't fully know what is and is not included on the DVD beyond the 4 episode commentaries. If Book 1 is anything to go by we will get the second part of the not so great Puppetbender short, but I personally hope that Republic City Hustle is included on the DVD as it is canon content and should not be a Blu Ray exclusive. The Comic Con panel seems like it is way too long to be included on the DVD .
I have the DVD ordered and when it arrives I will of course review it here for the site.
Both sets are available on Amazon and should be available in most places that sell dvds/blu rays.