Is bench press with dumbbells less effective than bench press with barbell, if so why?
In Starting Strength, a well-known book on barbell training, Mark Rippetoe says (emphasis mine):
In fact, the dumbbell version of the exercise, which actually predates the barbell version due to its less specialized equipment requirements, is probably a better exercise for most purposes other than training for a powerlifting competition. This is especially true if the weights used are sufficiently heavy to challenge your ability to actually finish the set. Most trainees use them as a light assistance movement, and never appreciate how hard they are or how useful they can be. They are performed on a simple flat bench, and taking the dumbbells out of the rack or off the floor, getting into position on the flat bench, and getting up with them after the set is finished is a large part of the exercise. Dumbbells — being not tied together between the hands as with a barbell - require more active, conscious control, are harder to do, and are therefore less commonly done. There is a one-handed version, in which only one dumbbell is used, that requires a tremendous amount of core strength to just stay on the bench.
It goes on to recommend doing the barbell version ("as the weight of history and precedent demands"), though.
It is a different exercise. Quoting Mark Rippetoe (Starting Strength, p.68):
... the dumbbell version ... is probably a better exercise for most purposes other than powerlifting competition.
He also says that getting into position, and getting up with the dumbbells after the set "is a large part of the fun", and that they require more conscious control and are harder to do compared to the barbell version.
I don't see why it should be less effective, except in regard to the bench press as competitive lift.
From my experience when Bench pressing with a good spotter will be 10 times more effective. it tends to use more of your core and also if done correctly you will see a vast improvement when training to your tri's too. You center of gravity is also better when barbell pressing and ensures a good press.
I would only dumbbell press if if the bench press equipment is not capable of spotting you with a lift your self and you believe that you could be putting your self in danger.