The dumbbell press works more of the arm muscle due to the fact that it makes the srms stabalize the two separate weights. The stabalizer muscles that it works are essential for strong punches. The bench press tends to not work a lot of these stabalizer muscles, but lets you put more energy into the press. You should altetnate between different excersises, so as to keep the body guessing and because each excersise acts differently.
Another great chest workout is the dumbbell fly. It fully extends the pecs (for max gains), and doesn't require much weight if done properly.
Now the shugyo begins!