Which file should I use - MP3 or OGG?

The short answer is:

The MP3's are variable-bitrate, which may mean they won't play on very old MP3 players. If you have a CD or flash-based MP3 player from around 2000 or earlier, read the manual to see if you'll need to convert the song.

The OGG/Vorbis files are both smaller and better quality than the MP3's, but they will not play on Windows or Mac OS X systems without a Vorbis codec installed.