You need both/all. The best fighters have developed from a combination of live experience, drills, and pell work. The pell is to teach you how to throw shots properly and to train all of those little muscles so that your shots become muscle memory and are automatically thrown correctly from that memory. It won't do you much good if you practice throwing them incorrectly. Use something heavier that will encourage you to use proper form (and help build those musskulls).
Fighters hit heavy bags, speed bags, pads, and so on, and they isolate various techniques and situations in training. If you want to excel in foam fighting, you need to take the same approach.