Db's are good, safe on the shoulder, but difficult to get a good (important) hard contraction of the pecs making the exercise a little inefficient.
cables enable you to really get a good load at the point of contraction, but the resistance falls away pretty fast as you stretch the pecs.
a machine can be real shoulder wrecker if the biomechanics of the machine is poor, if you're not sure about the machine don't use it.
but a good machine is the best movement for the pecs especially when you use a machine that has a lot of adjustment.