Depends on the machine and OS. On machines which support AES-NI (Sandy Bridge and newer) you should use FileVault. On all other machines AES-128 encrypted sparse disk images (create them with Disk Utility) are the better choice. AES-256 is much slower, and easier to crack than AES-128.
Bruce Schneier in Comments of Another New AES Attack said:
That being said, the key schedule for AES-256 is very poor. I would recommend that people use AES-128 and not AES-256.