If you are gonna play both, start with I. If you aren't interested in playing two similar titles, just play 2.
I definitely consider them more of a parallel evolution of the Zelda I model. They are more like Zelda I than L2tP. That doesn't mean they are simple, however. Character movement and interaction with the world doesn't feel quite as restricted as Zelda I, but the puzzles and dungeon designs aren't quite as clever. Also, the combat is more fast and furious. N2 is better than N1, but they are both VERY similar. N2 feels more like N1.5 in a lot of ways. Definitely more evolutionary than revolutionary.