I once owned both an Onyx,and then an Onyx Platinum.I don't believe they looked any different,at the time,except in my case the Onyx Platinum had a more beautiful Onyx body.Of course this is probably a unit to unit variation....But...
Sonically I DO remember the "PlATINUM" to be the better sounding design.
Hope this is of some help.No doubt you'll get more up to date info from some others.
Best