I've changed my mind.
Such reasoned arguments now lead me to say, stick to font grades. I would have said that normally, but for some reason was thinking earlier that if people feel strongly about it, put split grades in. However I was being daft. Both grades mean the same thing so just put one in, with Chris's stunningly innovative idea of a grade chart at the front to allow comparison.
Most poeple I know move seamlessly between the two systems, even mid sentence, so its not like we don't understand both systems. So putting both in would be a waste of ink and in these days of dwindling natural resources and planet destruction, we all need to do our bit. Ground Up could lead the way in this often forgotten, yet key environmental issue.