Q: I noticed iTunes Match is using solely AAC (*.m4a) type files when 'loading' files from the cloud. I'm a bit OCD about my collection getting mixed up with incompatible-type files with Apple's AAC format. Is there any way I can change this? Also, is there a good reason why they're using AAC in place of regular MP3s?
A: A visual comparison isn't truly a good way to explain something that requires listening to, but let's try it anyway. We'll start by looking at two graphs below comparing an AAC file vs an MP3 file, both encoded at 256kbps.
Can you tell which one has more embedded data? If you've answered 'Exhibit A,' the AAC file, you are correct. But as to the quality of listening experience, it'll be all subjective from there on out. That's pretty much it the idea though - Apple has chosen a format that is higher-fidelity than the MP3, allows producers more flexibility when compiling their music, and is actually a smaller filesize than standard MP3's.
Another myth is that AAC files are Apple-proprietary. This is untrue. It's actually an open format that's been developed by some time. Among other benefits are: different industry taxing schemas for distribution of the file-type (explains why iTunes Match can actually pull off what it does) and increased all-around battery life on iOS devices.
As for device compatibility, nearly all new playback devices today support the format, including gaming consoles, set-top boxes, and TVs with DLNA support, so you need not worry about your collection going backwards in time in terms of compatibility - it'll be just fine.