Cabin noise is there in both the cars.
But cultus is relatively more comfortable than alto,spacious,bigger legroom space and a good drive.
I have been using alto since 2 years.I have never encountered any problem.It all depends on the way you keep it,maintain it and drive it.
Well, I am a student and to me Alto is a good car.
But if you drive with family, then I will recommend you Cultus.