Complex sentence: The focus of the roadtest was to test the Marauder against its closest-and arguably only-rival, the discontinued Hummer H3; unlike the original Hummer H1, the Hummer H3 is a civilian model based on the Chevrolet Colorado.
Split into simpler sentences: The focus of the roadtest was to test the Marauder against the discontinued Hummer H3. Unlike the original Hummer H1, the Hummer H3 is a civilian model based on the Chevrolet Colorado.