Visable abs are the resut of two things;
1. Thickness of the covering fat layer
2. Size of the abs
1.
Reduce the thickness of the fat layer with a good diet and cardio. Low fat, carbs a little under what you are using and protein for rebuilding muscle and recovery
2.
Increase the size of the abs and they will become more visiable as the skin and fat layer covering them will become tighter and be stretched by them.
Bottom line..... work on both things at once. Good diet, decent amount of cardio above what you are currently doing, weight training. Weight training does not only have to be centred around the abs. A number of compound exercises will also help like squats and deadlifts. The compunds will also help strengthen the rest of the body and so help you keep proportional. Situps would not hurt though
. Tackling only one point will not get you what you want in the most efficent manner. Tackle both at the same time ***.
Cheers
RB