mingoox:
A couple of points to consider, as well:
1) Ogg Vorbis support with NAudio is simple: NVorbis (specifically NVorbis.NAudioSupport). Also on NuGet. The usage is generally similar to Mp3FileReader.
2) If you are in the U.S., and this is a commercial project, MP3 might come with baggage you don't want. Specifically, you might need a license to distribute the encoded files - OR - might need to use a licensed encoder to get the distribution license. IANAL, so please check with an attorney if you want to make sure.
Performance differences between NVorbis and NAudio MP3 are hard to say offhand... NVorbis isn't native code, so it has some CLR overhead to deal with. NAudio MP3 is partially native code (all the expensive parts), but pays the P/Invoke "tax" for the privilege. NVorbis uses larger buffers, but NAudio MP3 has to use multiple buffers. Benchmarks will be your friend here.
If you choose to use Vorbis, I recommend using the latest version of the aoTuv encoder.
Good luck with your project!