libfaac is a safe aac default, but aacplus works MUCH better at the the 8 - 16 kbps range, which seams to be a stated goal of this page. It turns on SBR+PS and has ffmpeg support.
If any Dropbox people are reading this, can you comment on what criteria you used to pick the audio codec?
If any Dropbox people are reading this, can you comment on what criteria you used to pick the audio codec?