GMAT OG Quant Review, 2nd Ed: Data Sufficiency
Like Problem Solving, Data Sufficiency hardly changed from the 1st edition to the 2nd edition.
Qualitatively, the new problems are very similar to their predecessors, testing fundamentally identical content. The phrasing or some other minor aspect may be original, and in general, the new problems are excellent additional examples of GMAT-like style and feel. So these problems are absolutely worth doing. Just realize that they are essentially "more of the same."
As far as topic distribution goes, Word Translations expanded somewhat, while other topics changed only slightly.
The 1st edition had 118 Data Sufficiency problems. 23 problems (19%) were removed, leaving 95 repeats. Then 29 problems were added, yielding 124 problems—slightly more than before.
The proportion of problems in various topical categories has shifted, but only a little. Word Translations has expanded the most. Changes in the other topic areas are slight.
Problems were replaced in the 1st edition at all levels of difficulty (as measured by problem number), but unevenly. More problems were replaced in the bottom quartile than in any other quartile.
Repeated problems did not change very much in number; in fact, many problems have identical numbers in both editions. This indicates that the additions and removals happened in roughly the same places in the sequence.
The average difficulty for various topics generally remained the same. Fractions, Decimals, & Percents problems rose the most in difficulty, but these problems were the easiest as a group in the 1st edition.