Looks like everyone is in general agreement. I've been to Maui several times and I participated in a Train to End Stroke marathon on the Big Island. I have been to Maui, Kauai, Oahu, and Molokai so I was excited to visit the Big Island. When I got off the plane, I was disappointed until I drove the entire island the day before the marathon. The area around Kona was mostly lava rock and kind of ugly unless you like lava rock! But the other coasts were much more beautiful. I think it is definitely worth seeing, but just does not have as much activity as on Maui.
Big Island:
Volcano
Drive around the island with stops along the way
Green turtles in Kona
Downtown Kona
Maui:
Drive to Hana - kind of fun if your child can stand to stay in a car seat for a while. We always continue around the island even though there were warnings for rental cars. The roads get a little bad and there isn't much between Hana and Wailea, but it's a beautiful drive.
Haleakala sunrise is nice - if you go to Maui first, plan to go there early in the trip when your body is still on the Central time zone since you have to be up very early to make the drive to the top. Bundle up!
Depending on when you're going, you might catch whale watching, which is fun.
We always stay in condos in Kihei, which isn't as touristy, but has nice beaches and no big waves (unlike the other side of the island).
Lahaina - cute touristy city
Maui Tropical Plantation - http://www.mauitropicalplantation.com
Restaurants to visit: Mama's Fish House (other side from Kihei where you can watch windsurfers) and Haliimaile General Store in Makawao.
Have a great time!!!