I think Additi was splendid in her scenes ...
and yes nandini might look vulnerable from the outside, she's a strong women from the inside which you can see from her scenes!
and the scene where Ani's recording is played i think Additi did an absolutely AMAZING job, so many expressions portrayed in one scene ... her sudden happiness, relief that finally she could hear him again, see him again but then in a split second all that was shattered and realisation dawned on her that he was gone, gone forever and would never come back! Additi did all those expressions amazingly, you could see the hurt/pain on her face when Ani said I miss you ...
Nandini is also a very family orientated girl, hence doesn't wanna hurt anyone, therefore tries to stay happy for them, like she didn't wanna worry biji ...
anyway people should stop comparing both the actresses, i think both are doing a good job and both are potraying their charcaters with full justice! they are different in their own ways and that has been shown by Additi and sanjeeda very well!
wishing both of them Good luck! :)