I spoke with customer service. The one priced $35 with 'Exclusive Content' isn't actually the same listing as the one for $27 on Amazon (or $30 at Best Buy), but unlike the 'Limited Edition Giftset' it does not include the 64-page excerpt booklet "Art and Making of the Dark Knight Trilogy," just the three movies plus a code for a download of the documentary "The Dark Knight Reborn," a 35-minute exclusive about the making of the trilogy with "additional behind-the-scenes footage". I would assume that's what they mean by 'Exclusive Content' but I can't say what exactly is in that documentary since I haven't seen it. Also it supposedly has slightly different box art.
After hearing that I just went with the $30 box set (same as this one on Amazon) because it just seemed like a better value. When you look at it, it was less than $10 per movie when you consider the special features discs and booklet thrown into the package. The value for the documentary download was still probably thrown into the $35 exclusive set at BB but you can still purchase the download for $15 if it really means that much to you.
If you go to Best Buy, they have the $30 and $35 sets, and the $35 sets are almost impossible to tell from the others except for a sticker that says it includes The Dark Knight Reborn and a brief mention of it on the back cover. Otherwise, the coverart is identical.
BOTH trilogy sets from Best Buy DO include the 64 page Art and Making of the Dark Knight Trilogy booklet.
I bought one of the $35 ones and can tell you, mine has both the Reborn documentary and the 64 page book. It is the definitive version (for now).