To discuss KJV or NIV “only” by laypersons is to presume a depth of knowledge which is difficult even for experts and translators. I like to read the preface of any Bible translation which I am using. I favor the KJV because that is what I grew up with it but I will consult the ESV, Amplified Bible, or other translations as needed in doing my study and research. The Dead Sea Scrolls and many manuscripts had not been discovered when the KJV was authorized. E-sword is a free download which has several Bible translations and other Bible helps. A few quotes from the NIV 2011 Preface might illustrate the difficulty of Bible translation.
2011 Preface to the revised New International Version of the Bible
“The complete NIV Bible was first published in 1978. It was a completely new translation made by over a hundred scholars working directly from the best available Hebrew, Aramaic and Greek texts.
The first concern of the translators has continued to be the accuracy of the translation and its faithfulness to the intended meaning of the biblical writers. This has moved the translators to go beyond a formal word-for-word rendering of the original texts. Because thought patterns and syntax differ from language to language, accurate communication of the meaning of the biblical authors demands constant regard for varied contextual uses of words and idioms and for frequent modifications in sentence structures.
For the Old Testament the standard Hebrew text, the Masoretic Text as published in the latest edition of Biblia Hebraica, has been used throughout. The Masoretic Text tradition contains marginal notations that offer variant readings. These have sometimes been followed instead of the text itself. Because such instances involve variants within the Masoretic tradition, they have not been indicated in the textual notes. In a few cases, words in the basic consonantal text have been divided differently than in the Masoretic Text. Such cases are usually indicated in the textual footnotes. The Dead Sea Scrolls contain biblical texts that represent an earlier stage of the transmission of the Hebrew text. They have been consulted, as have been the Samaritan Pentateuch and the ancient scribal traditions concerning deliberate textual changes. The translators also consulted the more important early versions — the Greek Septuagint, Aquila, Symmachus and Theodotion, the Latin Vulgate, the Syriac Peshitta, the Aramaic Targums, and for the Psalms, the Juxta Hebraica of Jerome. Readings from these versions, the Dead Sea Scrolls and the scribal traditions were occasionally followed where the Masoretic Text seemed doubtful and where accepted principles of textual criticism showed that one or more of these textual witnesses appeared to provide the correct reading.
The New Testament authors, writing in Greek, often quote the Old Testament from its ancient Greek version, the Septuagint. This is one reason why some of the Old Testament quotations in the NIV New Testament are not identical to the corresponding passages in the NIV Old Testament.”