TrueCrypt: You can view the source code (or pay someone to do it) and verify that the program is doing what it says it does, and that there are no backdoors. You also can control the type, strength, and cascade of encryption.
Your SSD firmware: ????
However, using your SSD firmware is going to give you greater performance.
On a practical level you would probably benefit more from using your SSD's firmware unless you are really paranoid.
Actually using the encryption: Based on this forum post (it covers the 320's but should apply for the 520's as well) - the encryption is automatically applied to the SSD NANDs using keys internally generated on the drive. If you pick a good ATA (BIOS HDD) password, you are OK. The only way to change that password is to issue an ATA "SECURITY ERASE" command to the drive - that forum post says in that event the SSD will regenerate its internal keys, rendering the already-encrypted data useless. Of course you are trusting Intel to make that assertion.