In regards to Alucard, he's more powerful than we think. Besides, he has centuries worth of experience. Don't forget about his blood healing ability. In theory, if he made it rain blood after casting Dark Metamorphosis and wearing a blood stone, he would pretty much be invincible. Add the Muramasa, the blood thirsty two-handed sword from SotN that grows stronger with the more enemies that bleed are killed with it, as well as a ring of varda or two, and he would be completely unstoppable. Then again, any individual who is either a vampire or of vampire decent would fit in to this.
As for Soma, as I said before, he has potential. The fact is that the power of dominance right now only acts as a defense mechinism and he looses the abilities he absorbs once he is completely out of danger. In all honesty, if he could make it so that he never lost the abilities he absorbed, then over time he would gradually become more powerful and likely eclipse Alucard in terms of strength and power, but Alucard will always have the most experiece aside from Hecter that is.