There is no difference.
An MP3 audio CD is a Data CD. And you can add a playlist using either option.
The reason we changed it is becuase your average user does not know that a Data CD and an MP3 audio CD is one and the same. To avoid this confusion and to help the first time users, we created an option for MP3 so a beginer who knows he wants to create an MP3 Audio CD does not need to know that a Data CD is the same as an MP3 Audio CD. They only need to know "I want to create an MP3 CD" and they see an option for this and can select it.
To take it one step further, when you select the option for creating an MP3 audio CD, we only allow MP3 and mp3PRO audio files to be added. ALong with playlist. We do not allow you to add data files.
You could of course go to the Data CD option, and select the MP3 files and Playlist and craete an MP3 Audio CD that way as well. But you could also add normal data files in this mode too.
Simply put, we did it to make it easier for the beginer, so they do not need to know that a Data CD is the same as an MP3 Audio CD.
PS. CDA format is for standard Audio CD's only.