Basically they are quite compatible in scale. The 21C are cheaper, but they sometimes have larger hinges and such. The 21C Pz38t, Marders and Hetzer are very nice little models. FoV tends to have more diecast in their construction, the molded detail tends to be a bit "crisper", but they also have a much higher MSRP. Fortunately, except for the usual Panthers and Tigers< there isn't much overlap between the companies. Both have produced some nice models that look good "as is" or can be updated with modest effort. The original FoV seperate figures were horrible, but the newer sets are quite nice. Just my opinions.
Gary B.