There is no "right" answer here without knowing where and when you intend to play and listen to the same media.
If you plan on listening to MP3 media across players in different settings, then using MP3Gain is your best bet. If you intend only to listen via your computer, then on the fly loudness equalization is fine, too, but does take processing power every time something is played (not much, but still).
It's really a question that can only be answered based on your actual intended listening scenario(s).