I never heard the difference between the 2 formats. I downloaded both versions of MPLSound and the only difference was the amount of time and space the WAV version required. Any knowledge (not opinion or speculation) to share on this?

Go with the WAV format. You could always convert it to an MP3 file later(that is assuming they don't have some sort of mechanism in place to prevent you from doing this, which I doubt). The WAV format is un-compressed while the MP3 file is compressed and lacks all the fidelity of the original content but gives you a much smaller file size. If I get an MP3 file at bitrate of 128Kbps, I can notice a difference in the bass. When you start going towards higher bitrates such as 192Kbps or higher you might not notice any difference. Then again, if you can't hear a difference now, it's probably not that important to you.