Legally...
Postado por Houssem Khaled em Espanha forum
When I went to do my NIE, they told me the NIE was valid for 3 months (even if you renew it, it will still be valid for another 3 months, etc.) while the resident card allows you to stay longer in Spain without having to go through the trouble of renewing it as often as the NIE. Can someone confirm that?