A lot of experienced bodybuilder a will claim that dumbbells are less beneficial and for higher reps and that the barbells are what is really important... Is this true?
Barbells lets you use significantly more weight than dumbbells because you don't have to use as much balance, very few people use dumbbells half as heavy as their barbells, personally, I can add about 25% of the weight I'd use with dumbbells in total.
This means you can put a heavier load on your triceps, pecs and shoulders (maybe), which in turn means more mass building.
Bodybuilders are more concerned with isolation work than strength athletes are. Body building, at it's core, is about body modification. With dumbbells you can get a lot more specific and target certain things in very specific ways.
Barbells shine in certain key areas however:
Squats/deadlifts/cleans. You can sort of do these with dumbbells, but not in any realistic way at big weights. With big compound movements that involve the largest muscle groups in your body at maximum force, the amount of weight you can move is simply too large to reasonably be found in dumbbells. IE, no one's deadlifting a pair of 250lb dumbells.
A big thing to hold onto. Again, for something like a snatch or clean, you can throw a lot of weight in the air and you need something big enough to handle it (like a barbell).
Incremental loading. Most gyms allow you to add 2.5lb (~1 kilo) weights to a barbell, allowing for 5lb (~2.2 kilo) adjustments. A lot of serious athletes bring fractional plates which allow you make adjustments at even smaller levels.
Really heavy dumbbells are a pain in the ass. The heaviest object you work with in barbell land is ~50lb. The plates never go over that, the bar never goes over that. So moving things around is relatively simple. With big dumbbells, it's not trivial to move a 150lb dumbbell around.
They're smaller, sort of. A stack of 300lb's of plates and an olympic bar fits in a smaller area than an equivalent rack of dumbbells.
Dumbbells are great too in different ways:
They're safer to bench. It's really not safe to max barbell bench press without a spotter. You can get really injured if you can't get the bar off you. With dumbbells, you just drop them.
You can do more isolation work. Personally I'm not a fan of body building techniques in general, but there's no argument that they allow for more direct targeting of particular muscles. My concern is that isolation work is the last thing a new gym goer should concern themselves with.
They're not as intimidating. Most new people stay away from barbells but dumbbells just seem more straight forward.
They develop more stabilization muscles and keep imbalances in check. With something like the overhead press, both arms need to get each weight up. You can't rely on your right arm to compensate for a bit of weakness on your left. Also, the (dumbbell) weight is simply less stable so you'll get more muscle activation across a broader range of muscle groups.
Flipping the explanation in the answer provided by Mârten, I believe the downside of barbells is that they could potentially prevent you from developing both sides of your upper body equally.
Put another way, a benefit of dumbbells is that they force both arms to work independently of the other. Working each arm individually may help you to develop the ability to lift heavier barbells.
So I thought the opposite was true, but to answer your question, this article from Men's Fitness says that each method has its advantages.
- Great for beginner safety
- Developing form
- Great for single-limb training/improving strength imbalance
- Have a higher maximum weight than dumbbells, therefore they are better for "serious strength training"
- Provide an even load to the spine when squatting
- Best for exercises which "require the entire body to move as a coordinated unit" (e.g. the clean or press)