In salah after surah fatiha we say Alhamdolillah and sunnis say ameen. Why the difference and what did the Prophet used to say? Alhamdolillah or ameen?

They consider the Fatiha as a dua, that is why they say Amin.
We have hadith which says say Alhamdulila after Fatiha.

Also, saying Amin is speaking in prayer and if it not done with intention of dua it can be considered as speaking in prayer which can invalidate the prayer.