The UBS4 Greek New Testament, in print, was designed to be used by translators. It provides information on major variants that may affect one's translation. Thus, the UBS4 apparatus is smaller, but where it does provide information on variants, it traditionally supplies a lot of information. The UBS4 has, in actuality, two different apparatuses -- the textual (or critical) apparatus and the discourse segmentation (i.e., punctuation) apparatus. In addition, a cross-reference system is also meshed in the apparatuses.
The NA27 Novum Testamentum Graece, on the other hand, is geared more toward scholars and textual critics who often have different concerns than do translators. Thus, the NA27 apparatus has information on many more variants, but it is presented in a more compact notation and often to less of a degree. It also treats variants that may not have translational significance (e.g., variants in word order or orthography). The NA27 also has a huge amount of information in the margins of the text (cross references, Eusebian canon references for the Gospels, and more).