If I have to select between Nihira and Himani, I would say that Nihira has the perfect voice and range for playback singing, but Himani should not lose her talent and make a waste on pop singing alone.
Himani can follow the track of Begum Akhtar, and Ustad Bade Ghulam Ali Khan and churn out her own albums which will surely have a market of its own. After listening to contemporary music which is too noisy and jarring, it would be a welcome relief to listen to some sound classical music and ghazals.