I want to spend a couple paragraphs explaining as best as a guy can who doesn't understand all the complexities of the issue but has a little light to shed on this important question.
A few different people in the last few weeks have had an epiphany of sorts and recognized that the NIV (and other modern translations) are missing verses in the NT. Where the NIV skips a verse, The KJV has the verse there. (For instance Matthew 17:21).
Many people make the claim that the modern translations are faulty or less accurate...or not the Bible because these verses are "deleted". Therefore the KJV is the only acceptable translation.
Here is what happened:
In the 1600's, the translators of the KJV set to task to translate the Bible to English. They did the best that they could with the resources they had, but their sources that they were translating from were LATE. Some of the texts were from 1000 years after Jesus' death and resurrection. In some more peculiar instances, they didn't have access to Greek at all and had to translate from the Latin Vulgate what they thought the Greek said.
What has happened in the last 400 years is that we have discovered over 500 sources that are MUCH OLDER than the sources used in the KJV. Some of them date clear back to 200 c.e! What scholars discovered in all of these ancient manuscripts is that certain verses were not present. These verses were inserted (Added) to the original manuscripts by scribes. (Keep in mind that the scriptures were copied by hand to be passed on for centuries...there were no computers, thumbdrives, etc.) The KJV (to no fault of the translators) translated these verses that were not originally present in the text. The KJV is an inaccurate translation because of this. It has verses that are not part of the original manuscripts.
The NIV, NASB and NRSV are all utilizing these BETTER RESOURCES in translation. Therefore verses that weren't there originally have once again been removed to try to capture the authentic, true nature of the scriptures.
So Which Translation is the Best?
The NIV is an okay translation. It was translated by a team of conservative, evangelical scholars. This results in a slight conservative, evangelical bent on some passages.
If I recall correctly, the NASB is very wooden in its translation. It tries to translate the verse, word by word. When a translation does this too rigidly, a lot of the nuances of the text are missed. If there is a euphemism or pun in the text, it misses it. The NASB sometimes misses the heart of what a text is trying to imply.
The NRSV is the best translation available. It was translated by a scholarly group that consisted of evangelicals, pentecostals, roman catholics (i think?), and jews (?). This eliminated the bent to one type of bias. They also sought to maintain the heart of what a text was saying while maintaining its integrity as well.
Is this helpful? Would you add anything? Disagree with anything?