Personally, I would choose Broomfield over Superior any day. Oh, wait...we did. Let's take a look at your criteria:
Good School System - Boulder Valley is top-notch. Monarch might have higher CSAPs but it is anything but a typical "neighborhood" school. Think more like..a mall or a prison.
Family-Friendly - Broomfield has numerous family-friendly activities. Not only is the Derda Rec center a great place (with its indoor aquatic park, indoor playground, Kids Nite Out program, and excellent child-sitting area), but Broomfield also has the outdoor water park, The Bay. A fun place for the whole family.
The Mamie Dowd Eisenhower library and attached Auditorium (the Audi) is also phenomenal. The library has lots of year-round activities and some really fun summer programs for kids: Wonderful Wednesdays for school-aged children and Fun Fridays for preschoolers.
I could go on and on about the family friendly activities but they are too numerous to list! If you're around the weekend of Sept. 14-15, drop by Midway Park for the community-wide Broomfield Days celebration.
Also, we still spend a lot of time in Boulder, despite moving away two years ago, and a lot of families there drive their kids out here to participate in our youth sports program (baseball, football, lacrosse, soccer)
Shopping - Flatirons is very accessible from Broomfield.
Arista is also currently under construction and will add ~600-800k additional square feet of retail to Broomfield.
Things to do with children under 5: see above
BrianPhil is right about Flatiron Crossing's proximity to Superior but he overlooks the importance of its location. Because Flatiron
is in Broomfield, our town retains the revenue generated by Flatirons, which means better amenities for Broomfield residents.
Newer housing: The Broadlands is a cookie-cutter subdivision and I think there are more new homes being built further east but we choose to stay west of Sheridan because of school district. I think BrianPhil may live in my neighborhood, which is very diverse but is a perfect fit for us. We're in the second filing, which is older but within walking distance to Midway park, Boulder Valley schools (elementary, middle, and high), the library and auditorium, the future civic center, shopping, 2 mi from Flatirons and the future light rail station and 5 minutes from Arista and the Broomfield Events Center. We are 20 minutes from downtown Denver and 15 minutes from Boulder. A similar house in Boulder with comparable walkable amenities would have cost us 500k+.
A lot of people like Superior but we do not. It is a town with no vision. There is no planned light rail station, its retail options are not comparable to its neighbors (Louisville, Boulder, Broomfield) and its largest subdivision Rock Creek, which houses 97% of its residents, has radiation from Rocky Flats. It's your decision where you choose to live, but you should have all the facts. .