The two sentences come down to the same thing - a woman is explaining to someone why she does not think that she will make (probably him) a suitable (marriage) partner.
But whether she uses the words in your first or second example would probably depend on the context.
If the couple were merely speculating about their future, they might well use the first example. She is explaining that she doesn't think they should consider marriage because she believes it would be an unhappy relationship.
But if the man had actually asked her to marry him, she might well use the words in the second example. Here she is explaining why she is declining his invitation, saying that she will not marry him.
These are merely typical situations in which either statement could be used in either context. But the first is more likely to be used in a hypothetical situation, the second in a real situation.