WMA files give better quality sound then MP3 files for the same size, therefore unless there is a specific reason to convert them I would leave them as is. (Converting to MP3 will just make the files larger if you want to maintain the quality)
AAC would probably be a better choice as it is the improved industry standard alternative to MP3 and part of the MPEG standards, and has wide support amongst players.
I would not recommend WMA as it is a proprietary codec and support is limited compared to other formats.