Filevault or Truecrypt?
Truecrypt's main selling point is that it runs on windows, various flavours of linux and  OSX, too. From their website, it says that they offer many encryption techniques each with a minimum of 256-bit encryption.

FileVault uses XTS-AES 128-bit encryption, which is a different form of encryption, but despite it's misnomer actually has 256 bits of encryption.

FileVault is tightly integrated into the OS, so it's probably easier to use. If your use case is over many computers with different OS's on them (windows, linux, as well as mac) then I'd go for Truecrypt for continuity.

It's important to remember that the security of these drives is limited by the length of your password. This article has an interesting discussion for further reading.


  Does Truecrypt decrease the performance of the OS comparing to Filevault?


Without any testing whatsoever, I would guess that they're comparable in speed, since they are both on-the-fly encryption techniques, and perform as normal drives once unlocked.

EDIT:
I forgot to mention that TrueCrypt also supports encrypting just a portion of your hard drive, say a folder or a group of folders. 

Filevault doesn't support encryption of individual files/folders. However, Disk Utility will allow you to create an encrypted image. (which can use either 128-bit OR 256-bit AES encryption)