Translation:She likes sausage and does not like mayonnaise.
Wow, that's an amazing question.
I'd say because in this case we are not concentrating on the statements being opposite.
If I say it like this: "Вона любить ковбасу і не любить майонез" I will definitely say i. I am making a stress on the actual facts and on the food
I can also say: "Вона любить ковбасу, а не любить майонез". In this case I will say а because I don't mind that much about delivering information, my focus is on the like-dislike part. Maybe prior someone said it wrong, the other way around, that she likes mayonnaise, and I am disagreeing and emphasizing the correct fact.
But this is my native speaker guess :D I'll think more about it... Wow, really, cool question.