I am only familiar with the Continent based RTW fare on 1W. If you use one of those fares, when you come through the US, you will be charged a price for 5 continents (EUR/Asia/SWP/SA/NA) rather than 4 if you skip the US connection.

As for IB vs BA. I have heard that BAs seats are more comfortable than IBs inter-continental seats. I have no personal experience on IB long-haul.