Only you know how niche is your case. If there is a chance of denied boarding, logic says you should book with the carrier that has multiple flights (BA), in case you get denied boarding on the first flight and you need to be rebooked. (unless IB can easily re-book you on BA). All in all BA may be slightly more competent around the issue and in general, but MAD is IB's main hub, so, hard to tell.