The major difference between the KJV and the NKJV, is that the King James Version 1611-1769 (the Authorized Version, so called) was written by the Bishops of the Church of England specifically to support the Ecclesiology and doctrine of that Church. American versions of the KJV are different, and not authorized to be read in the CofE. The Queens publishers are authorized to print the only version of the KJV approved to be read in the Liturgy.
The NKJV is an American publication that was written using similar language to the KJV (to become a modernized 17th. cent. English), but which was not written by Bishops of the CofE, and not approved to be read in the Liturgy of the CofE. It is a Bible that is translated by two Cambridge scholars in the first instance - one a very scholarly Bishop, the other a very distinguished Greek translator. Their work became the basis for all the post Darwinian Bibles that we have today - I use this term because it was the publication of Darwins theory (1859), that forced theologians to strengthen their position on Creationism, which incidentally led to the discovery of two ancient Greek Bibles (Sinaiticus 1859, Vaticanus 1859), one in the Vatican archives where it had lain since the 11th. cent., and one at the Monastery at St Catherine's on Mount Sinai, where pages were being used as fire starters. Vaticanus was the primary text used by Westcott and Hort to make a translation into modern Greek in 1881. This translation was used to produce the English translation called the Revised Version, which spawned the American Revised Standard Version, and countless others including the NKJV (1962) and of course the NIV.
Now, that is the background to the NKJV, and KJV. Here is the advice on which to choose.
I have 28 Bibles for reference (electronic of course) but I tend to stick to Bibles written before 1881 (the date of first publication of the RV NT). The most useful is the Authorized version of the King James Bible printed by the Queen's printers - by the Oxford printer for the USA. Almost all the 19th. cent. commentaries and Lectionaries (Strong's etc.) are keyed to the phrases in this Bible. So, when you read the commentaries you can look the verses up and find the same words in the Bible.
The other Bible is the Duoay-Rheims English translation of the Vulgate Bible which was written in Classical Latin in the late 4th.cent from Old Latin manuscripts primarily, but compared to Hebrew Torah, and the Greek Septuagint from "Origen's Hexapla" of the 3rd. cent. King James version (AV 1769) and the DRV teach the same theology.
The word order and sentence structure of Bibles after 1881 are different enough from the standard Commentaries (of which there exist more than a thousand), that it makes it difficult to grasp the significance of the comment, when the text of a particular Bible is worded differently.
One thing to be wary of is a "study bible" because often the commentary is geared to the doctrine a particular religious group and then drafted into another Bible where it is not relevant.
Note: Bibles have been commissioned to support particular doctrines. You cannot use a Roman Catholic Bible to support non-Catholic doctrine. And Protestant Bibles cannot be read in the Catholic church Liturgy, nor in Anglican liturgy ("liturgy" applies to the order of Sunday Service.)
For an introduction to this, go to my non-commercial web site.