Your question:
“Some guys in the gym always do dumbbell bench presses for chest, but a trainer told me that the barbell bench press is the best way to stimulate the pecs. Who’s correct?

We had 10 highly trained athletes participate in this study of muscle activation through EMG analysis. We measured muscle activity in the upper and lower pecs as well as the front delts. The athletes performed both the barbell bench press and the dumbbell bench press using 75% of their one-rep max (1RM) for four repetitions.

The barbell bench press involved the upper pecs and front delts more than the dumbbell version, while the dumbbell bench press hit the lower pecs to a greater degree.

When you grasp the bar using a wide, fixed hand placement (as you would on the barbell bench press), bringing your hands together at the top of the movement is not possible. This keeps the emphasis on the upper pecs and delts. In the dumbbell version, however, your hands are free to move across your body, and this causes the increased involvement of the lower pecs.

If you traditionally do both barbell and dumbbell presses for chest development, keep it up. The combination of both barbell and dumbbell work increases muscular development throughout the whole pec and shoulder area, giving you the most balanced chest possible. If you typically gravitate toward only one, however, you should begin incorporating both into your routine to get the most out of your chest-training efforts. From week to week, as you vary your working angles, try alternating between using the barbell and dumbbells on exercises.