FastTokenizers are implemented in Rust and are factors faster than the Python based tokenizers. Apart from that their encoding methods should behave the same. However, they are not functionally identical. From the docs 4:

When the tokenizer is a “Fast” tokenizer (i.e., backed by HuggingFace tokenizers library 5), this class provides in addition several advanced alignment methods which can be used to map between the original string (character and words) and the token space (e.g., getting the index of the token comprising a given character or the span of characters corresponding to a given token).

Typically you want to use the fast tokenizer if it is available.