M4A is AAC in Apple's favorite outfit: the audio-only MP4 your iPhone speaks natively. It was born in 2003, popularized by Apple's iTunes Store, and its real weak spot is older Windows software and stubborn car stereos that expect a .mp3.
WMV is a time capsule from the Windows XP years: tidy little files with dated manners. Windows Media Video, usually inside an ASF container; once the backbone of DRM movie rentals. That makes it the right destination when what you need is old Windows archives and PowerPoint decks that still insist on it.