I read from this website that they are interchangeable, but for some people, "eccetto" is more formal
In another exercise, I was asked to translate 'except the wine', I put 'tranne il vino' - it said I was wrong and that the correct answer was 'eccetto' (hope I spelled that right BTW) and now I see this exercise the other way around and it gives 'tranne' as the right word for this sentence. I have read all the posts here and basically know which to use and have an Italian friend who has given me their opinion. I'm just saying how frustrating it is to be 'wrong' when I wasn't in effect, I don't think. Still a wonderful language-learning tool though, with loads of wonderful, helpful people.