My students in China often have this problem. Don't think so hard, a simple response is good! You can just say "Thank you" or "Thanks." This will sound much more natural than either of the sentences you wrote. If you want to be more specific, you could say "Thanks for your concern." However, this sentence is very formal, and if you say it to a friend, you will sound cold. Still, it is okay to say to a stranger or to your boss.