The short version
For many first-time readers, the NLT feels easier because its English is especially natural and conversational. The NIV is also very readable and is common in churches, study groups and general-purpose Bibles. If your church regularly uses one of them, that may be the simplest place to start.
How the reading experience differs
The NLT tends to explain the sense of a passage in smooth contemporary English. The NIV generally sits a little closer to the structure of the original wording while still aiming for natural English. That difference is noticeable, but neither choice should turn into a panic.
Which is better for devotional reading?
If your main goal is to read larger sections without getting stuck on sentence structure, the NLT may feel especially approachable. The NIV is also strong for daily reading and may be easier to follow when sermons, classes or friends quote from it.
Which is better for study?
Either can be used for serious study, especially alongside good context and more than one translation. If you begin comparing individual words closely, remember that no English translation reproduces every feature of Hebrew, Aramaic and Greek in exactly the same way.
A practical way to decide
Read the same chapter in both translations before buying. Try Mark 1, John 1, Psalm 23 or James 1. Notice which version you understand without constantly rereading. The translation you consistently open is usually more useful than the one you bought because someone declared it perfect.