Phalasamapatti is a meditative state in which a person is absorbed in Nibbana itself.
In this attainment there is a special kind of consciousness present called Lokuttara-citta.
In contrast to this, in Nirodhasamapatti there is no consciousness at all.
In Chapter 8 section 42 of the Abhidhammatha Sangaha it describes it like this:
Having proceeded, thus, up to the base of nothingness, one then attends to the preliminary duties such as the resolution, etc., and enters the base of neither-perception- nor-nonperception.
After two occasions of javana in absorption, the continuum of consciousness is suspended.
Then, one is said to have attained cessation (nirodha-samāpatti).
Another difference is in who can attain them.
Phalasamapatti is attained by each of the four kinds of noble beings just after attaining the knowledge of the path, and it can be cultivated and extended by them as well.
Nirodhasamapatti however, can only be entered by non-returners and arahats.