Its a tricky choice. Im ACA. My opinion is that if you can get into EY or Deloittes, its a great CV move and opens doors later. In truth, i think it can be harder getting into EY etc later in life than it is to get into a smaller firm so consider taking the opportunity when you can. I now work in a smaller firm BUT I still think that having had large firm experience, it puts me in a better position when pitching etc etc