If it is a morning flight I would say BA since breakfast will be better. IB's breakfast is not as good as BA's. However, assuming you are going to Madrid, note the BA flight arrives at the terminal 4 satellite which means a train ride to the main arrivals area. From 11.35 onwards (first IB flight not serving breakfast) I personally prefer IB. Better food and wine and faster exit from the airport. Service can be good or hit or miss on both, depends on the day and the crew.