Elliot et al. 1989, Ferreria et al. 2003, McCaw and Friday 2004 all demonstrated by EMG that pectoralis major activation (clavicular portion) for dumbbells and barbell bench press is the same. Anterior deltoid activation during these exercises was similar as well. Interestingly, tricep activation was not measured in these studies and might bear investigation. All these trials used 10-12 rep max. This would indicate that at sub-maximal levels, the primary movers of the bench press work the same way regardless of the medium used and it would in fact be the stabilizers, which are accounting for the difference. One would suspect that as loads approach a 1 rep max, activation patterns would differ since barbell bench press loads are much higher- however one should reserve an opinion until the study is completed. Thus, you can substitute one for the other in your training and for those muscles get similar results.