We made our decision based on who we thought would bring up our children in the most similar way that we would. My parents are still fairly young (in comparison to DH's parents who are closer in age to my grandparents) and I was brought up with a very similar parenting style to what we use for our kids. So we have chosen my parents. I know that one of DH's sisters would probably be a little upset that we didn't chose her as she is single and adores kids, but her parenting style would be wildly different to ours and we didn't want that for the kids.