The Sport only has low gear (more gear reduction for more torque; doesn't overheat while crawling), so it won't be able to go as fast as the one with the two-speed.
More or less, a combination of both. If the vehicle has an ESC that doesn't modulate well, the car would be jumpy on low-speed starts, which can make even an experienced driver struggle on steep inclines, since the sudden starts and stops can break the balance of the car. On the other hand, if...