I have a Border Collie and knowing what I know about them now, I would definitely buy a Golden Retriever if you want a dog that is calm and will be happy with a daily walk.
Border Collies are VERY energetic dogs who require lots and lots of exercise. They love to run off leash and just never seem to get tired. They get bored easily and are very strong willed. Yes they are smart, but often this can be a disadvantage because they can outsmart their owner and get themselves in trouble.
Both breeds are good around children and neither one is aggressive, but a collie will have to be taught not to nip at them or try to herd them like they would a flock of sheep. It's instinctive for them to do so and it needs to be trained out of one very early on.