If (and with me that is always a BIG if :D) I have my facts right, 256 AAC is the equivalent of 320kbps MP3. In which case they are both the highest level lossy format audio. The only thing higher would be WAV or lossless formats. The latter not always supported by all DJ software.
I think more important is having EVERYTHING you use in the same format. I’d rather have everything in 320kbps MP3, than stuff mixed with WAV, MP3, AAC, etx.
You can always keep a copy of high quality originals in a separate place (external storage, online storage) and then run a copy through iTunes Match ending up with all your files in the same quality and format.
Greetinx,
Chuck