I feel like Chevy is marketing the hell out the Cruze, atleast in my area. When it comes to their cars, they are promoting the Camaro and Cruze the most. The Sonic is kinda just an interesting "what's that in the corner" car.
Whereas Ford loves to promote the praise the Fiesta and Focus have been receiving. Even Hyundai and VW love to promote their hatchs.
Interestingly enough, on the Camaro board I belong to, one of the members (who sold Ford products) complained that GM spends too much money on ads compared to Ford.
This came up when Chevy got the Motor Trend Car of the Year award for the Camaro and the Truck of the Year award for the Colorado. He claimed that GM was "buying" he award. Of course I then reminded him that Ford has won the award before too - so Ford must have "bought" them too...
Ford did have a Focus TV commercial last year, but not one for the Fiesta.
It makes sense for Chevy to promote the Camaro as it's an icon car. The Cruze is a much more profitable car than the Sonic and it attracts more people. The Cruze outsells the Sonic by nearly 4 to 1.
Advertising is done to get people on the lots. Once they get there, people buy what they buy - often by how much they can afford.
If the Cruze ad gets someone on the lot and they end up buying a Sonic, GM is still happy.