lets see, i assume you will be going to boston from the United Kingdom.
this means you will have to connect via Paris or Zurich, so based on that you are saying Air France or Swiss.
hmm, i think the connection through Paris will take less time.
what sort of aircraft does Air France fly? probably a 772 or an A340
swiss flies an A330 it looks like.
i think Air France is in the middle of updating their first class or business class cabins, or both can't remember which.
either way...you're going to be immensely comfortable, either way, you'll be hearing french.
since you are in the UK and travelling to the USA, i would think that Swiss would be a better choice.
here are the reasons.
1) they're in Oneworld. you live in the UK, you could use your miles on BA
if you like.
2) you are going to the states, if you need to do any personal traveling in the states, you can use your miles with American Airlines.
3) you might catch an A330 down to Zurich, which means you will have real first class as opposed to the narrow body psuedo business class that Air France would operate.
so i would go with stay. if you are lucky...perhaps you will even have the fortune of finding an A343 or M11 to Boston to Zurich!!!!! Now wouldn't that be a treat!
hope this helps...
PARIS, FRANCE...THE BEIRUT OF EUROPE.