As a native, I can be certain that both pronunciations are correct.=]
In classrooms, students are taught with 'neih5'. This pronunciation is generally considered as 'proper' by the field of education. However, 'leih5' is as well a correct pronunciation despite many think it is a result of sloppiness or laziness (懶音). In fact, the choice of 'neih5' is completely arbitrary. For more details, please read pg.4 of Cantonese A Comprehensive Grammar by Stephen Matthews and Virginia Yip.
For the sake of learning, I recommend you to speak 'neih5' instead of 'leih5' because speaking the former will give locals an impression that you're taking the language seriously. But no matter what, Cantonese is not a highly structured and well documented language as English, so be prepared for confusions and enjoy its dynamics.