Along with other notable advantages, learning genuine French in France with a correct accent and all the colloquialisms that go with it is the biggest one. It only makes sense that to speak correct French you need to learn in France. There are other very good course offered in the UK, Canada and the US, but in order to speak as they speak in France you will hold a definite advantage by study there.
Many French courses offered in the country also give insight to the country where French started and also give you the opportunity to learn about the culture. You can learn about the history, culture and geography of the country with other students creating the best way to learn any new language. Surrounding yourself with those interested in the same outcome.
Talk about economic issue, enjoy the music they enjoy, watch French TV, eat their cuisine and learn how to realistically order food at a restaurant. The French course in your own country will teach you the grammatically current way, while classes in France give you hands on.
If you were to live in France or study in France for weeks or for year it obviously gives you a better overall understanding of the country, its people and the language. Learning French from the French is better than any other method. There are always problems associated with learning any language.
Like many languages French has many dialects with each having its own vocabulary. It is not as simple as accents like the difference between the north and south in the United States. In this case the languages are nearly completely different. This is comparable to the Cornish and true Yorkshire in the UK. Just within France there are many different languages spoken, for instance, Gascon, Lorrain and Languedocien. Lest we forget classical French.
Learning French while living in France make sure it is true French and be thankful for the chance to learn other dialects if you so choose. If you choose not to learn the local dialect you can focus on just what is known as classic French, not colloquial.
If you take local classes close where you live, you will sign up and you and the teacher probably know very little about the language and country, nor what version you are learning. Your accent will be atrocious and if you went to France, very few would understand your regular school French.
Therefore the advantages of learning French in France are superior to anywhere else. Not only will you learn to speak but you will understand when spoken to.