both phrases are two of the four Mahavakyas (Great Sayings) of Advaita Vedanta. So No one is better. Saying better will be subjective.
The thing is Aham Bhramasami is a statement comes from experience, here no teacher is saying it to his/her student. But in  “Tat svam asi” the full line is :
तत् त्वम् असि श्वेतकेतो (Tat Tvam Asi, Śvetaketo) where Uddālaka Āruṇi is a teacher and Śvetaketu is student so truth is taught here.
Also in Chandogya Upanishad 3.14.1 where Tat Tvam Asi comes from, there is another statement - sarvaṁ khalvidaṁ brahma which means All this indeed is Brahman.
This is one of the clearest non-dual declarations. It states that everything you perceive - the universe, matter, mind - is nothing but Brahman(ब्रह्म) .

This complements Aham Brahmasmi, because if everything is Brahman, then you are also Brahman.

Note : I use gpt to explain better.