The three most common ways of doing a bench press are with a barbell, dumbbells, or Smith machine. Norwegian researchers, using a technique called electromyography, found that muscle activation differed during the three types of bench presses.
For the triceps muscle, activation levels were greatest for the barbell, followed by the Smith machine and dumbbells. Activation levels were similar between exercises for the pectoralis major and anterior deltoid muscles. Biceps activation was greatest during the dumbbell bench press. Each exercise was effective for building the major upper body muscles, but each provided slightly different effects.
Source: Journal Sports Science, 7: 1-6, 2011