The main difference being TrueCrypt creates containers and 7-Zip encrypts the file itself, so file sizes can be guessed. Now let's just talk about the strength and breakability of the encryption.

Update: http://forums.truecrypt.org/viewtopic.php?p=107396

If implemented correctly, AES is AES; the output between two different implementations is identical, and therefore no distinction is possible in after-the-fact comparison -- if done correctly, the one is exactly the same as the other.

But there are a few points where differences can crop in:

Operation Mode
Truecrypt implements a modified counter mode called XTS. It's pretty well vetted and has withstood some serious abuse from some powerful attackers (such as the US Government). 

From examining the p7zip source code, it appears that AES encoding for the 7-zip format operates in CBC mode. This is certainly not necessarily insecure; it's the mode most popularly used in protocols such as TLS, but it is potentially vulnerable to padding oracle attacks. See this discussion on operation modes for more information.

Key Derivation
Truecrypt uses PBKDF2 to turn your password into an encryption key. It's difficult to come up with a better alternative than that. p7zip uses a salted SHA256 hash repeated over a configurable number of iterations. PBKDF2 is a bit more configurable, but 7-zip's alternative is functionally similar and arguably reaches the same goals.

Vetted Implementation
Here's probably the biggest difference: TrueCrypt's code has been poured over by cryptographers and carefully examined for implementation mistakes. 7-zip's has not (at least not to the same degree). This means that there is a higher probability that 7-zip's code contains some sort of mistake that could allow for some sort of as-yet-unknown attack. That's not to say that such a mistake exists, and that's not to say that such a mistake couldn't be found in TrueCrypt instead. But the this is a matter of probability, not certainty. 

All in all, the differences are minor, and for most use cases you shouldn't expect any difference at all from a security perspective. If it's a matter of life-and-death, I'd probably pick TrueCrypt. But for matters of mere secrecy, I'd recommend going with whichever solution fits your problem the best.
