Dual citizenship: Croatia and U.S.

I have dual citizenship: Croatia and U.S. After working for long time in the U.S. I recently relocated and work in Croatia. Last year when I was living and working in the U.S., a Chinese Institute invited me as a short-time scientific visitor and I went getting visa as a U.S. citizen. Now when I am in Croatia, they are inviting me again for a short visit (week) within a month or so. Which passport I should use: Croatian or U.S. or it does not matter which I use?