Which Cartop Cargo Box?

I know that a car top cargo box will impair MPG, but I think it's better than the alternative: driving our Sienna. This is particularly true for trips in which we need cargo room to and from, but not during. As such, we've a Thule rack ready to accommodate a cargo box. I'm leaning towards the Thule SideKick which has 8 cubic feet of space (and costs $199). The Ascent 1500 has 15 cubic feet of space, but is considerably more money ($359).

Any recommendations? The reasons for the extra cargo is trips in which my DH and I are traveling with our 2 daughters (ages 4.5 and 7 yrs.) and our dog (who does better in the cargo area than in the seat with the girls).

TIA!