All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Both recitations are valid (authentic). Al-Jazari stated in Al-Nashr fi al-Qira’at al-‘Ashr (2/309):
“There is a difference regarding the recitation of ‘Laqad ‘Aalimta’. Al-Kisa’i recited it with a Dammah (an apostrophe-like shape mark) above the ‘Ta’ (‘Aalimtu), while the rest of the reciters recited it with a Fatḥah (a diacritical mark) above the ‘Ta’ (‘Aalimta).” [End quote]
As for the speaker (the person who is meant by the speaker’s speech), it is Musa (Moses, peace be upon him) according to the recitation with Dammah, meaning: “I am certain that what I have brought is revealed from Allah.”
As for the recitation with Fatḥah, the person who is meant by the speaker’s speech is Fir’awn (Pharaoh), meaning: “You (O Fir’awn) are certain that what I have brought is revealed from Allah, but you denied it out of arrogance.”
Allah knows best.