Skill Focus: Concentration gives a +3 bonus on all concentration checks. Combat Casting gives a +4 concentration check only when casting defensively. If your caster were only going to take one of these feats, and prereqs for prestige classes are not involved, wouldn’t Skill Focus be the better option because it is useful more often? The difference between them is only +1 and Skill Focus is going to help on any check for concentration, whereas Combat Casting is only useful for the one situation - choosing to cast defensively. Skill Focus will increase checks for Casting Defensively but also for damage taken through attacks of opportunity, for readied attacks against the caster, taking continuous damage, for tough conditions like casting on a moving ship’s deck, casting in high winds, or even just being distracted by a hot elven babe that walked into the room during casting.