If you barrage into the same point that you use for artillery to hit 4 buildings then from my experience barrage deals slight more damage to each bullding than an artillery but it is inpredictable and may actually cause less damage to one of the buildings. So if you shoot like that then once the artillery becomes I'd say 20-30% more expensive than barrage then it is better to use barrage.However, if you use two barrages and shoot them like on the pictures below then you are making more damage. I usually use the first variant. There often quad defenses on Gearheart bases. You can practice that and see what will come up the best.