Ogg is better than wma at mid level bitrates. However, wma is better at low bitrates. At higher bitrates MPC beats them all out.
Frankly, just use whatever is easiest to implement. Maybe you can take the ogg decoder and "copy and paste" it into FS_open.
The biggest reason why there aren't many ogg hardware players is because up until relatively recently, there wasn't an integer decoder for ogg. Since it require floating points, hardware players can't support it. Now that an integer decoder has been created, hardware players supporting ogg are appearing.
However, mp3 is definitely ubiquitous and most people wouldn't understand what ogg is and why they should use it.
In factm most people won't be able to hear the difference anyways (one way to tell is people who think the ipod has great sound quality XD)