I am very sure Ishu will fall first...because she is in peace with her past. She is hopeful and positive.Most importantly she doesnt have a closed perspective like Raman that she cannot falll into love.Though even Raman will fall quickly but he will take much longer to realize.So the realization will first come for Ishita.
And that makes sense acc to story as well coz once Ishu falls for Raman she will be ready to sacrifice anything for him...even her own love.Staying away from Ishu will make him realize her worth.