I prefer dips great chest exercise and tri killer. Al had a good point though, if you want that cut line it's gonna come down to diet.... you can see tiny chested guys with very low body fat with a cut line under the pecs.... however I think his statement came out sounding a little sided... you can grow bigger pecs by training them and then you'll also have more of a cut if they are bigger, but diet an cardio will play big roles even then!!
and yes declines "do" something... any exercise does something.... just most prefer to trade out a better exercise ie dips for a lesser form of exercise, training the same muscle groups... ie decline