I really like single mag carriers for CCW. They give you more options. Now, for a range or comp holster I think doubled or quads are better because they are more stable on the belt. For an IWB I would go single.
This^^^^The biggest issue with IWB double mag holders for double stack mags is that you're really starting to need a lot of extra room! I can get away with wearing an IWB holster inside of most of my various pairs of pants but things start to get pretty tight when I start stuffing mags IWB, too.