Whether Combat or City Raider is preferable has nothing to do with a unit's base strength. Anyone who thinks so needs to learn how the combat system works (and then write angry messages to Firaxis for wanton cruelty to common sense, but it's a little late for that). Attackers' bonuses from Combat promotions are applied directly. Difference of summed other bonuses is applied to whatever side has more. Combat can give better odds than City Raider or anti-unit-type promotions when one side (usually the defender) has a very high bonus. Often, Combat beats City Raider or Cover against Archers... unless it's on a Quechua.