Is it true that Serpent and twofish are much stronger than AES? I heard that Serpent and Twofish are much stronger than AES, but it was chosen because it is faster. If that's true, please tell me why it is stronger.
Also: is it true that Twofish and serpent are better protected from brute force attacks because they take longer to en/decrypt?
SHORT: This is kind of true. However, things are bit different now. Better protection against brute force is inaccurate claim.



At the time Rijndael (AES) won the competition, it was faster, and sufficiently strong. After the competition, Rijndael (AES) has gotten faster (AES-NI and other hardware improvements). Also Rijndael (AES) has also gotten significant amount of analysis, but it has not been broken in a practical way. Therefore, Rijndael (AES) has been shown to be quite good pick.

The other AES finalists are also believed to be very good, but as they've not been as standardized they will lack interoperability. Also, we've gotten very good in making Rijndael implementations during the last two decades. Because Serpent and Twofish have gotten less attention, it may be harder to find as good implementations of these algorithms than it is to find very good AES implementation.

The difference in en/decryption speed between these algorithms does not translate into any meaningful difference in protection against brute force attacks.

For more information, see also earlier answer by B-Con to similar question regarding advantages of Rijndael against Twofish and Serpent at: https://crypto.stackexchange.com/a/5290.