Kingdom of Croatia vs Empire of Serbia

Who would win ? The greatest Croat state with almost all of it's vassals (I had to compromise since both states had similar territory in some areas) and it's strong navy/large army and competent fighters or the short lived empire that rivaled even the Byzantine Empire. The Serbian capital is closer to Croat's reach than Croat's capital to Serb's reach, but the Serb army is also capable and they're an Empire afterall.