I don't know about the Casio comparison but the Modcan 58B VCDCO rocks for FM!
http://www.modcan.com/bseries/modulesb.html
I got one of these a couple of weeks ago and can't stop playing with it. If you like FM sounds I
don't know how you could possibly be disappointed with this module. It's not cheap but it's
half the price of a ZO! There are demos on the Modcan site and I plan to upload a few of mine
in the next couple of days - check the Yahoo Modcan group.
Bruce really cares about his customers too, like Dieter :-)
Phil