There is a lot of "audiophile" misinformation in this thread. Very few individuals can A/B accurately above 256kbps in any sort of compression, and certainly some have better ears than others, but that's the general consensus for threshold where the high quality is difficult/impossible for most people to distinguish.
A bigger issue is related to the OP's equipment and if they're even good enough. Fortunately, it seems he has some pretty great entry-level speakers (depending on how you look at it), but they are definitely good enough to tell the differences between lower and higher bitrates. In practice, I am doubtful the OP will be able to distinguish between either 320 MP3 or 256 AAC, but the simplest solution would be just to A/B it to see if it makes a difference. On a technical basis, AAC might be the better choice, if marginally, and again, I have my doubts as to whether there will be any real-world hearable difference for the OP.
On principle, I use ALAC for all of my recordings, even though I know that lower quality rips have a negligibly perceptible (if at all) difference to my ears, but if I can only get something on iTunes or in 320 MP3, I'm not even remotely beat up about it. Let's be honest here, when you're talking at these levels of compression and fidelity, regardless of your equipment, the minute compression artifacts and errors are on such a low order of magnitude that they will not reduce your listening enjoyment at all. Again, I'm talking about that "threshold"; lower bitrates are easily discernable to even the untrained ear and for me, unlistenable. Part of why I hate Spotify and Rdio.
tl;dr: OP, try out both and see if you can tell a difference. If you can't, and chances are by limitations of biology and probability, you can't, then don't worry about it. If you're completely bonkers like me, then just get everything in FLAC/ALAC.