I'm all for mp3's but m4a's are great too because they hold the "explicit" tag for iTunes. My iTunes tends to match the clean versions of mp3 songs 90% of the time. Plus apple isn't perfect by any means but for iTunes Match, mp3's that get uploaded instead of matched tend to error and only play 2/3 of a song before skipping to the next on phones.
I've found that I have to convert 320 mp3's to m4a and manually adjust the tag before I can drop them into iTunes. They usually match, but if it's a leak, they'll "upload" and I definetely lose some quality there.
I know there's other music programs out there but for apple users this tends to be pretty common.