Utility
The CX-9 will very likely prove more practical than the HUMMER H3 for those of you with frequent child-cargo.
The Mazda CX-9 is likely a better choice than the HUMMER H3 regarding transporting cargo, especially if you frequently need to carry a lot of equipment or large pets.
Towing is substantially more the HUMMER's forte than it is the CX-9's.
Dimensions
The HUMMER H3 is a bit heavier than the Mazda CX-9.
Your wallet will hurt more every time you fill up the HUMMER than the CX-9 because of its larger tank.
The H3 and the CX-9 compete for the same parking spaces.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. It costs about the same to get the HUMMER to the dealership as the Mazda CX-9.
As far as MSRP, the Mazda CX-9 costs approximately as much as the HUMMER (though the HUMMER is somewhat cheaper).
Handling
The HUMMER H3 and the Mazda CX-9 have approximately similar tires.
With their comparable turning radii, the HUMMER H3 and the Mazda doubtless handle roughly the same into and out of tight spots.
Drivetrain
The Mazda CX-9 and the HUMMER H3 have similar-sized engines, with a slight advantage to the HUMMER H3.
Comfort
While the front cabin in the H3 offers a bit more head room than the Mazda CX-9, there frankly isn't much of a difference.
The CX-9 provides more head- and leg-room to your passengers than the HUMMER H3 does.
Overview
The basic warranty that comes with the Mazda CX-9 isn't quite as generous as the H3's.