Theoretically the 320 MP3 will ALWAYS be better. But that is purely in theory...
Is that a personal theory? If bit rate were the only variable, yes, 320 would be better. But there are other variables. When there are other variables, "ALWAYS" goes right out the window.
For example, is a 9 GB episode of
Fringe better than a 7 GB episode of
Fringe? Absolutely not if the first copy is MPEG-2 and the second copy is MPEG-4 AVC, even if from the same original. The critical variable here is that MPEG-4 is a better codec; it can either do a better job at the same bit rate, or an equivalent job at a lower bit rate.
Its the same with AAC vs. MP3. AAC is a better codec, and can also either do a better job at the same bit rate, or an equivalent job at a lower bit rate.
So the OPs question, which is better, MP3 at 320 or AAC at 256, is still an open question. 320 is absolutely not ALWAYS better.
With all else held equal, it was well accepted back when Apple started using AAC that a 128 kbps AAC was equivalent to a 160 kbps Fraunhofer MP3 (other versions of MP3 were inferior). If the math is linear, that would make a 256 AAC equivalent to a 320 MP3. But then development on MP3 has stagnated while there have been a number of improvements to AAC since Apple first started using it in 2003.
That would tend to support the theory that the 256 AAC Apple ripped from a CD would actually be
better than the 320 MP3 that the OP ripped from a CD, assuming the AAC used Apple's most modern flavor of AAC. And you can probably bet that Apple is not just using the consumer encoder available in every free copy or iTunes to build their library, but top-shelf encoding hardware. That equals even better quality.
And if Apple really is getting the masters, which is their stated intent, that will be significantly better. One of the basic rules of encoding is that when you restrict to a severely-low bit rate in a lossy format such as consumer AAC/MP3, what really makes a difference is the quality of the original. And that is exactly why they are doing it.
Problem is, I don't think that program is all that ramped up quite yet, and it will probably take a few years for them to really get it pervasively through their library (which is why although I intend to do iTunes Match, I am waiting).
A CD is 16 bit at 44.1 Khz. That's an equivalent uncompressed bit rate of 1,411.2 kbps (for stereo). Modern masters are 24 or even 32 bit at 192 KHz, which can give them an uncompressed bit rate up to 12,288 kbps. This means more samples more often, up to a factor of times 8.6, or even better. It also means quantization steps orders of magnitude less far apart. That means a whole lot fewer or smaller rounding errors during final encoding, which means a whole lot fewer audio artifacts when compressed at an equivalent 256 kbps bit rate.
We might actually get back all of the things we have given up for the portability of MP3s, including stereo imaging, separation, real crisp highs instead of swishy highs, and real solid bass instead of mushy bass or bass MIA. Maybe even some dynamic range.