Here is another great reason to use Logos 4. The SBL Critical Edition of the Greek New Testament is available free. Michael Holmes edited this critical edition. Logos has tagged the links to lexicons and parsing guides work perfectly.
According to the preface, there are some 540 differences from the standard editions. This text as a data base the 6,928 variation units, disagreed with Wescott and Hort 879 times, Tregelles 1227 times the NIV Reader’s Edition 616 times, and Maurice Robinson and Willaim Peirpont’s Byzantine Textform 5959 times. Starting with Wescott and Hort, Holmes worked through every variant with these three editions and evaluated each instance. Holmes explains his method in detail in his article, “Reconstructing the Text of the New Testament,” in The Blackwell Companion to the New Testament (ed. David E. Aune; Oxford: Wiley-Blackwell, 2010), 77–89.
This is not a critical edition like the UBS4 or NA 27. Holmes does not cite manuscript evidence, rather he collates editorial decisions. For example, in Eph 2:11, the SBLGNT reads ποτὲ ὑμεῖς, following the WH Tregelles and NIV Reader’s GNT, against Robinson and Peirpont’s Byzantine Text which flips the word order. The NA27 text does not list this as a variant, but does report the replacement of Διὸ at the beginning of the verse with δια τουτο in F G. This does not appear in the SBLGNT presumably because all four critical editions agree to reject the evidence listed in the NA27. Neither variant appears in the UBS 4. The value of the SBLGNT is in collecting these four GNT versions into a single source. I would rather evaluate the actual textual evidence myself, so I will stick with the UBS and NA texts, but for many readers Holmes’ method will be enough to show what variants exist.
An XML version is also available, with an iPhone app listed as “coming soon.”