By choosing to become a Samana (ascetic), is Siddhartha taking a forward step towards his goal? Was not his life as a Brahmin's son better?

In my opinion, Siddhartha is following his heart. His life as a Brahmin's son is likely more appealing to us, but for Siddhartha, his position leaves much to be desired. He is looking for fulfillment, a spiritual awakening..... a purpose. In order to achieve this, his journey was a necessity.