It totally depends on context. In this sentence, "in addition" doesn't really work in the way you've used it. In fact, I'd say the following: "Florence is known for the Uffizi Galleries as well as the Academy." Using "in addition", you could say: "Florence is known for the Uffizi Galleries in addition to the Academy," or "Florence is known or the Uffizi Galleries. In addition, it is known for the Academy" Or, using "as well": "Florence is known for the Uffizi Galleries. It is known for the Academy, as well" As a native speaker I can only really tell you what sounds right, but the following may be helpful: "as well" is far more common than "in addition". "As well" tends to come at the end of the sentence, whereas "in addition" tends to come at the start. More examples: "In addition, I'd like to point out that the cliff has an excellent view of the sea" "I'd like to point out that the cliff has an excellent view of the sea, as well" "He has great fashion sense in addition to his good looks" "As well as being an accomplished writer, John dabbles in music" "He loves dogs as well" "In addition, he loves dogs" I'm afraid it's just one of those things where it depends on the context! I hope these examples offer some insight, maybe someone has a good rule for this.

Definition of (as well) as well as : and in addition : and <brave as well as loyal> Simple Definition of addition : the act or process of joining something to something else : the act or process of adding something mathematics : the act or process of adding numbers : something or someone more that is included : something or someone that is added