The two phrases are about the same. 낫다 is generally a bit more specific about comparison than 좋다 (i.e. 낫다 = 더 좋다), but if you have 훨씬 which emphasizes the difference, they become practically the same. By the way, unlike 좋다, 낫다 is also a verb which means to be cured of an ailment. So you can say something like 감기에 걸렸었는데 이제 다 나았어요 (I had a cold but I am fully recovered now).