I would think you could never plan for that kind of thing. It's like having a second ONLY because you want the other one not to be lonely. Three families i know did that and the siblings hated and still do hate each other.
As for me I had one of each, and you know what , I would say now I'd keep them, but I'd love an older brother and/or sister (Im the oldest). Are we EVER happy!!!! xoxoxo