There has been an ongoing argument for hundreds or even over a thousand years as to which of the ancient manuscripts to use. The difference between the KJV and the NIV is which ones the translators choose to use. When in bible studies, I encourage having both of them because there are clearly some differences between them that need to be discussed.
However, the New Living Translation is not a translation but a commentary. It will leave out scripture or completely change it to get their point across. If we are going to move that way, then I would recommend the Stubby translation because it only uses words of six letters or less. Or maybe the Free translation because it never includes the word not. Or how about the Rap the translation that only includes verses that rhyme.
In addition to rules of textual criticism which would indicate that the older reliable manuscripts be given more weight than the newer ones, and the obvious changes in the English language over the last 400 years, (for example the verb "let" then meant to hinder or restrain, whereas now it means to allow), there are other reasons that the KJV might not be the best translation. Let me explore one of them now.
The KJV was one of the earliest English language translations, (the Geneva Bible gave it a run for the money). As with any work of this magnitude, mistakes are going to be made. Part of the reason for subsequent versions is to correct those mistakes. Also, improvements can be made to insure that those reading it are not misled by a complex sentence structure.
For example: Many people have used 1 Cor. 11:27 translated thus from the KJV:
27 Wherefore whosoever shall eat this bread, and drink this cup of the Lord, unworthily, shall be guilty of the body and blood of the Lord.
to say that we must be worthy to participate in the communion.
Now technically there is nothing wrong with this translation. Unfortunately, many people are weak on grammer. The word "unworthily" is an adverb. As such, it cannot modify the noun "whosoever". Instead, it modifies the verb "shall eat". This fits perfectly with the context of verses 20-34 where Paul makes it plain that he is talking about the way in which the communion is done, and not about those participating. The NIV properly translates it "in an unworthy manner".
How many people have used this verse improperly to not participate in the communion as a result of the KJV translation? The NIV translators anticipated that some would take this verse in an ungrammatically correct way, and phrased it so it would be understood correctly.
Let me throw this one out to the once saved always saved crowd before you argue this one. How can we be in a saved condition, and not be able to commune with our Lord and Savior, Jesus Christ?