As far as translations go,
both are very nicely translated, for the most part.
As one poster replied, The NIV is a protestant (although with Catholic
and Jewish scholars as well) translation, while the NAB
is primarily a Roman Catholic translation (although protestant and Jewish scholars also
helped out in the translation of the NAB !!).
Most NIV "Study Bibles" have study notes, explanations, introductions, and footnotes
which reflect a nonCatholic doctrinal slant (symbolic-only Eucharist, often "once saved, always saved," especially in the Harper Study Bible, a chiliastic viewpoint in some study bibles, non-priestly "elders" only, no hierarchy, no other sacraments, etc.
The NAB, on the other hand, has many study notes that I find detrimental to
the orthodox Roman Catholic faith. That's the ORIGINAL NAB. I have to confess
that I have not read the study notes in the new and REVISED N.A.B. that came out
just a while back, so I cannot speak about that with any knowledge.
But the translations of the biblical text, in both versions, is pretty darned good.
The NIV has one drawback: It is missing the 7 Deuterocanonical Books found
in ALL Catholic and Eastern Orthodox Bible versions and in ALL Bibles prior to the
16th century, when Luther and later Calvin,** jettisoned them based on their own, personal, purely-self-appointed authority which was really their own **private and
**non-authoritative views **as to their "worthiness" of being included in the canon.