Both are gorgeously exotic and pretty. I think I would choose Indira, which is ironic because I have a very nice uncle named Nadir. He is named NadirAmir. You may also want to consider Amira.
IndiraI adore Indira, it has a beautiful sound and a great namesake. To me Nadira doesn't have the same flow. And it reminds me of nadir which can have a negative meaning.
Hard to choose. They are both beautiful names. I think maybe I'm leaning towards Nadira. Indira has been on my long list for a while now, but I hadn't seen Nadira before until just now.