The only game I've been extensively working on is Relics of Nature's Fury, and I try to use OGG files for that. Keeps it compact, and RPG Maker can read 'em just fine.
But yeah, if you're going for maximum compatibility, I'd say do what you've been doing. Which would be releasing both MP3 and OGG.
It's like what OCR does with their albums, releasing in both MP3 and FLAC; you can choose which one you want more, but in the end it's about catering to as large a crowd as possible. If you do both MP3 and OGG, you should be able to do the same basic thing.