difference between BitLocker 128-bit and 256-bit encryption?

BitLocker Drive Encryption supports 128-bit and 256-bit encryption keys. Longer encryption keys provide an enhanced level of security and are less likely to be successfully attacked by the use of brute-force methods. However, longer keys can cause slower encryption and decryption of data. On some computers, using longer keys might result in noticeable performance degradation. You can use Group Policy to change the length of the encryption key used by BitLocker.

In addition, BitLocker supports a Diffuser algorithm to help protect the system against ciphertext manipulation attacks, a class of attacks in which changes are made to the encrypted data in an attempt to discover patterns or weaknesses.

In this version of Windows, the default BitLocker encryption is AES encryption with 128-bit encryption keys and Diffuser. You can use Group Policy to select encryption without Diffuser.