I much prefer BA for the south american routes, but have done IB many times normally due to price.

The extra 40TPs each way are handy if you are chasing status, and it is almost always a lot cheaper than BA.

To me the seat is better on BA as it goes completely flat, and BA staff tend to provide better service. Sometimes IB staff can be a bit indifferent towards passengers. If you've flown QF long haul, then you'll find it's the same seat.

The food is better on BA most of the time, but they do normally have a nice Rioja onboard IB long haul routes.

MAD is a good airport but the change can take time, so make sure you leave enough time for your connection in the airport and don't hang around too long!