The NIV is closer to the original languages than the NLT. It is more "literal" and the NLT is more "dynamic." A translation committee decides upon a translation philosophy to try to adhere to consistently throughout the Bible. If you wanted an English translation that was a word-for-word translation from the original languages, you would look at an interlinear Bible, but to understand it, you would need to know Greek grammar (or use the Bible publisher's notes indicating the grammar). If you wanted the translators to handle the grammar for you so that it was more like English, then you would go for the NASB (I don't know where the NKJV lies in the literal-dynamic scope.)
If you wanted a translation where the translators converted Bible references that required some knowledge of the culture of Bible times into modern equivalents, you would look at a very dynamic translation like The Message (which some people would say is not really a Bible, but can nevertheless be very helpful to some). The problem with dynamic translations is that they are the farthest from the actual words written/spoken in Bible times (some being God's actual words). The literal translations are the closest, but are more difficult to understand.
Personally, I am of the opinion that using the translation that is the closest to God's words is best (in Hebrew, Aramaic, and Greek), and then learn how to understand it (an ongoing effort). However, if that is going to be so difficult that you make too many errors or won't want to pick up your Bible, then go with one that you will.
If you do not have trouble understanding the NIV, I would go with that. I like the NLT, but use it as a way to understand a passage when I am having trouble understanding the NIV translation of it (sort of like a commentary). There's nothing wrong with having several copies of the NIV. You could always give away the one you use the least if you wanted to. However, if it were a study Bible, it still could be something you would want to refer to in the future.
