Oakley has a guide on their lenses. I used those to pick lenses.
I think the Fire Iridium increases contrast while the Black Iridium is neutral, so all things being equal, I'd go with Fire Iridium. I think they're both mostly bright sunny day lenses though.
Second this, although I actually used my black iridium lenses at night the other day and it wasn't too bad, but the runs were pretty well lit.