The problems with m4a can arise with big/long duration files, also mentioned at http://www.anythingbutipod.com/forum/showthread.php?t=68532
I think somewhere Sandisk even owned up to the limited nature of the m4a support in the zip but I can't find it right now.
I dump to disk BBC Radio 3 programmes, which are 320kbps aac and some are very long programmes - a live concert is usually a minumum of an hour and can go over 3 hours. File size of between 150MB and 200MB is normal and over 500MB is possible. The Rockboxed Clip+ could play the file but very often, actually usually, stop abruptly or skip. The Fuze+ just plays them start to finish with as much seeking back and forth as you like and no excitement except the music. Clip+ would sometimes skip on really big flac files as well. I guess that 90% of the stuff people listen to is 2 to 5 minutes long with a few exceptions so the problem might never arise. I have lots of those but also a big collection of much longer lossless tracks and quite a lot of really large lossy files so I encountered the issue a lot and it matters enough that I would not choose another Clip+ or spend money on a Zip.
I hope Sansa did fix the noise of the Clip Zip as the Clip, Clip+ and old Fuze could be pretty poor. Did they really fix it so there is even no noise on button presses like changing the volume? I've read enough times that the Clip Zip hardware is almost identical to the Clip+ (except radio and screen) and I've bought two of those older players and both were noisy, one really bad, and I preferred to risk my money on the unloved Fuze+ because it has different hardware, and I'm really pleased I did as I have a player that plays the music, no noise, no surprises and no fuss. Probably tomorrow it will eat all my data and then burst into flames in my pocket but for the moment I feel this is much the best player from Sansa (if Rockboxed) and ridiculously cheap.