selecting a male or female puppy?

We have selected a sheltie as our new puppy but can't decide on a male or female? What are some of the differences?
Our breeder suggested a male, we have twin boys who are 7. We also have a male cat and don't want them "marking" all over the house, as a friend has suggested that this might happen.
HELP!

Frankly I think there are more differences from dog to dog than gender accounts for. I would go, and pick the puppy who appeals to you most, is not too shy, as that can be a problem with some Shelties, and that likes your sons!