San Diego. I live around the Vegas area and there is way more to do in SD with a child than Vegas. Save it for an adults only trip! Vegas has some fun things to do on it's outskirts if you like nature (Mt Charleston, Lake Mead, etc)-- but SD is better with a family hands down!
PS- if you do decide Vegas and end up at lake mead, or any water anywhere for that matter, make sure your child wears a life jacket! I had to work a 2yr old fatality this year on the beach and it was heartbreaking and easily preventable...