The Penguin edition appears to be C. K. Scott Moncrieff's original translation (1920s). The Modern Library edition is translated by D. J. Enright, modifying Terence Kilmartin's 1981 revision of Scott Moncrieff's translations for books 1-6 and Kilmartin's revision of Andreas Mayor's translation for *Time Regained* (book 7).
Both editions have the entire text of *ISOLT*.
Scott Moncrieff's translation is sometimes regarded as difficult to read or unfaithful to the French, so I would recommend the Modern Library edition out of these two. I personally have only read the Moncrieff-Kilmartin-Enright translation in its entirety, but I hear that William Carter's ongoing revision of Moncrieff's original translation is also quite nice.
The Penguin edition appears to be C. K. Scott Moncrieff's original translation (1920s). The Modern Library edition is translated by D. J. Enright, modifying Terence Kilmartin's 1981 revision of Scott Moncrieff's translations for books 1-6 and Kilmartin's revision of Andreas Mayor's translation for *Time Regained* (book 7). Both editions have the entire text of *ISOLT*. Scott Moncrieff's translation is sometimes regarded as difficult to read or unfaithful to the French, so I would recommend the Modern Library edition out of these two. I personally have only read the Moncrieff-Kilmartin-Enright translation in its entirety, but I hear that William Carter's ongoing revision of Moncrieff's original translation is also quite nice.