I was saying "sad face" in regards to the person she was arguing with. If you really think the smile isn't completely happy looking (she's not sporting a huge grin or laughing or anything), the best explanation is what it says in the Ultimania about how the discussion is on what will happen when they die. If it's anything sad at all, it's bitter sweet, but only because as I said, Tifa doesn't want to die. Neither does Cloud. If they must though, there's something happy that will come out of it.
Personally, I only see it as sad because of the context of the dialogue. Not that Tifa is specifically looking sad or defeated, but that these two people just confessed and explored their love for one another, and now they might die. Sure they might see their friends, but their fates are out of their hands and they just have to hope they make it.