I used the new SBL GNT, comparing it with NA 27, just to take a look at it. It left me with some questions as to methodology. The SBL website didn't reassure me any.
From the "Introduction" - The SBL Greek New Testament (SBLGNT) is a new edition of the Greek New Testament, established with the help of earlier editions. In particular, four editions of the Greek New Testament were utilized as primary resources in the process of establishing the SBLGNT (Westcott-Hort; Tregelles; A Reader’s Greek New Testament; and the Byzantine Textform 2005).
Frankly, I don't follow it. It seems as if the philosophy was, each of the four editions got one vote, and the text was established by a majority. "Where all four editions agreed, the text was tentatively accepted as the text of the SBL edition..."
I'm badly over-simplifying, nevertheless - WH didn't have the MS information that is available today, so is it right that they should have a full vote? Same with Tregelles. The Byzantine text represents a different school of thought, right or wrong, from the critical text, so their textual decisions are based on different criteria.
In other words, if someone accepts the premises of a critical text, then shouldn't Nestle-Aland 27 have the final say over at least WH, Tregelles and the Byzantine Textform? Why compare...well, it's not even apples and oranges, it's an apple and an orange and also a couple of really old apples.