Both sentences are correct, but they are used in different situations. If you say “お客さんが中国からきます,” the fact that some guest is coming is the new information you want to convey to the listener. "From China" portion is optional; the listener may or may not know about that part. If you say ”お客さんは中国からきます", you and your lister already know that someone is coming. So that's old information. But "coming from China" part is the new information that you wanna tell. By the way, "どれは正しいですが。" should be "どれが正しいですか"。 When you have a subject in question, it's always marked by が because that's the new information you want to know. Consequently, the response should be "--~が　正しいです。"

“お客さんが中国からきます”：有客人从中国来的。（重点在“有客人”的事实） ”お客さんは中国からきます“：（在有客人的共识上）他是从中国来的。（重点在“从哪里”来）

どっちでもよいと思っています ただ強調している部分が違います。