Croatia has more history, but Serbia seems quite capable of catching up. Right now no strong contender appears to be looming on Croatian horizon (boy, can I go tacky or what?), so for the next (quite a) few seasons, I think Serbia will have the upper hand. (Also add the WTA to the equation, if you like.)
for sure Croatia
MArio deserves to be in TOP 10 at leaset!
Karlovic is TOP 20 player
Cilic is material TOP 30 player for sure
Ljubicic is still player which deserves to be in TOP 30
Karanusic is TOP100 player
Right now Nole singlehandedly beats aynthing Croatia has to offer... present and past. I mean, I liked Goran a lot but he was mostly serve... Nole is more of a complete player, and I'm quite sure he won't stop at just 1 GS like Goran.
Serbia has the best player (Djokovic), but Croatia is MUCH deeper. So if I assume what the question is getting at, which is which country is better at tennis, I would say Croatia. But again, Serbia has the best PLAYER. It's a horribly phrased question.