IMO daughters should go with the dad and sons should go with the mom.
If the relationship had left bitter feelings, if kids went with the same gender the parent may teach hateful things. Also, they're IMO more likely to teach opposite gender how to treat/be treated by the opposite.
Most Helpful Girl
Don't split up the kids. It's hard enough having the parents split up without making the kids live in separate homes. Either they stay with one parent together, or they go back and forth together. It's not the kids who are getting divorced.
As for who they 'should' go with, I know a single dad who successfully raised two kids (boy and girl) into responsible, well-mannered teenagers, even though their mother was nasty and unstable. Back when the divorce happened, the courts insisted that the two toddlers should go with the mother, and she attempted to kill them. Even then, the court that the dad appealed to for custody decided that the kids would be better off with the mother's family members rather than their own dad. (This was when it was the norm for divorced mothers to keep the kids.) He finally gained custody and did a great job raising them. So, I think every hard-and-fast rule will have exceptions, and it's best not to have a certain 'policy' or 'rule' about who should have the kids without evaluating each situation separately.