I think the Germans will pick clay. That's the weakest service for Croatia #1 player Ljubicic and although Ancic's clay court results looked impressive this season, the only good quality clay court player he defeated is Robredo in a 5 setter this year RG.
And I still think on average, Germany is better on clay than Croatia.
What about Davydenko? What about Horna, Ramirez-Hidalgo and Garcia-Lopez? Consider that, in the limited number of clay tournaments he has played, he has collected 600 ranking points.
Anyway it's not just about the results. LOOK AT HIS GAME. Have you seen Ancic play on clay? If you have then you would see that he's very comfortable on the surface.
When the ball bounces up high into his hitting zone, he can strike the ball beautifully off both wings. If you watch his eyes, you can see how focused he is every time he hits the ball and, hence, he can cope with dodgy clay bounces better than most players. He is a smart player who knows how to construct points on a clay court and when to approach the net to use his excellent volleying ability to finish off many points. For a tall guy, he also moves and defends exceptionally well on a clay court.