I'm from Spain and both sentences are utilized without distinction. The only difference is that you can (but you don't have to) use "a propósito" with the term you are refering to. For example: "A propósito de Steve. Ha estado esperándote todo el día" or "A propósito, Steve ha estado esperándote todo el día" or "Por cierto, Steve ha estado esperándote todo el día" In english the translation to this terms would be: Por cierto - By the way A propósito - Regarding... The difference betwen english and spanish is that "A propósito" can be used alone. I hope it helps you to understand the difference between both sentences.

No, I don't think there's a difference in the usage of these words in Latin America or Spain, but "a propósito" can also mean "On purpose" whereas "por cierto" is literally translated as by the way.