I've always loved firearms, and I've always owned them, but I am now recently married and I have a new daughter. My question is if you could own only one firearm to protect your family what would it be?

Right now the weapons that I am considering are a tactical pump shotgun or a 40 s&w. I've owned 9mms in the past, but I don't know if I would feel safe protecting my family with one. I am really intrigued by the 10mm, but I have no experience, and it is hard to find people that own them to get their opinions. Anyway I appreciate you guys giving me imput in advance.