incline and decline work your chest from diffrent angles. its mainly for bodybuilders trying to develop diffrent parts of there chest.
just stick with the flat bench press. If your too weak after that then your doing it right.
with your bench press do 5sets of 5reps, then next time do 3 sets of 10reps then change every workout.
if your arms weaken 1st then widen your grip, this will use more of the chest.
if your chest weakens 1st then bring your hands together, so that your chest and arms weaken at the same time.