I agree that fighters are just way better than bombers. Bombers do slightly more to cities, but your cannon units are way way better at taking down cities. Because planes (for most of the game) are limited to attacking from your cities, you generally have cannons in reach of the cities your bombers can attack any way. Plus your cannon units don't have to worry about air defence and don't take oil. A fix would be to up the city killing power of bombers by a large margin.