Complete Arcane continued the "complete" line of books, which is to say, a few new base classes (not prestige classes but rather new base classes, like cleric, wizard, beguiler, etc), a few prestige classes, feats, and spells.
The Complete Mage, of which there will also be a rogue book coming out next month, I think another cleric one, etc, is actually a book telling you how to be a better mage, an extensive look at different concepts and what the typical best choices would be from the various other books, along with your standard feats and spells. It also has some additional arcane places or whatever, which I had no idea until I read the DMG2 is actually an ingame mechanic and not just like free miniplots or something.
I think the new line of complete books are more like what people wanted when they bought the original complete books. For example I barely look at the Complete Arcane book when I build a wizard because it mostly has "new" concepts not things specifically to help me be a better wizard (or sorcerer or bard for that matter). The Complete Mage is fantastic as a tool for helping you to build your character.
Richard M.
Grimfang"Whisper"Xaeraes