To start, I'm not a KJVO supporter.
I however wouldn't base any doctrine on a NIV or NLT over a KJV.
Here is the big difference...
The KJV is the only version that is translated from the Masoretic texts.
All other versions (NIV, NLT, Amplified, anything) come from something called the Alexandrian texts.
What's the difference between the two?
The Alexandrian texts do come first. The earliest copies we have are in about the 2nd century (don't quote me, I'm not up on my dates) and the the Masoretic text earliest copies are something like 1000 AD. So the Alexandrian defenitely predate the Masoretic.
However...
The Masoretic texts were written by Jewish scribes, who had meticulous methods of copying documents. I mean, it was their job and life, to copy things. They would do things like count all the letters in both manuscritps and if the middle letter wasn't the same, they'd burn the copy, no matter how many years went into making it.
Alexandrian texts on the other hand were a Greek translation of the Hebrew scriptures (what the Old Testament was originally written in). Alexandria was a thriving place for paganism and other such crap. Because of these Alexandrian texts we get such things like the deuterocanonical books. These books were never in the orignal scriptures and were only put in at Alexandria.
Also, the Dead Sea scrolls that were found, which contain fragments of all the books of the Old Testament, except Ruth, are almost perfectly matched to the Masoretic texts. There are very very few minor differences between the Masoretic texts and the dead sea scrolls. The dead sea scrolls are dated to about 100, 200 BC. So they predate the Alexandrian texts.
So all in all, the Alexandrian texts have been corrupted, but really only canon wise, and not content wise (if you know what I mean?). However, I would take the Masoretic texts (KJV) over the Alexandrian (NIV) any day.
Peace
P.S. - I should also mention that the Catholics don't use either of these manuscripts but rather a latin translation called the vulgate. Around 400 AD.
There are more than a few inaccuracies in what you say.
First the Masoretic Text is the authoritative Hebrew Text of the Old Testament. The Masoretes were the Jewish scholars who included vowel points in the text and, thereby, set the meaning of some passages that were otherwise obscure.
The Alexandrian texts are Greek New Testament manuscripts. The charge of corruption against the Alexandrian texts is made by KJVonly supporters. It is mostly a baseless claim used to deride the eclectic text of the United Bible Societies Greek New Testament.
The Vulgate is, indeed, a Latin translation of the Scriptures. It was prepared by St. Jerome and its Old Testament was based, I believe on the Septuagint, a Greek translation of the OT. The Septuagint included Greek writings that were not found in the Hebrew texts. These are included in Catholic editions of the Bible under the title "deuteroncanonical" books. It should be pointed out that, while modern Catholic Bibles still follow the scholarship of the Vulgate,
they are based on the same Greek and Hebrew manuscripts as other Modern translations. A quick read of the introduction to any Bible usually reveals its textual basis.
As for the differences between the KJV and NIV, they are twofold. First is the underlying Greek and Hebrew texts. The NIV uses an eccentric Greek text which is close, but by no means equivalent to the UBS text I mentioned above. Most other modern translations (with the exception of the NKJV and a few others) use the UBS text.
The KJV NT is based on the so-called Textus Receptus which included a number of scribal errors and additions. The KJV OT is based, as Smilebomb says, on the Masoretic text, but so are most of the modern translations of the OT. Even the Catholic editions reference the Masoretic text.
The other difference is translation philosophy. The KJV was translated in a very literal fashion. In a fw passages, it is so literally translated as to make little sense. This in no way diminishes the value of the KJV.
The NIV is much less literal in its style of translation. It seeks to preserve the meaning and thoughts of the original rather than word for word equivalence. I like the NIV for sheer readablity, but I don't consider it a scholarly resource.
Does this mean that the NIV is evil? Hardly. Many of the arguments made by the KJVonly advocates are specious at best. The KJV is no more perfect a translation than the NIV. It is only different.