They should all get the same priority as they all brike the same law.
Now, obvuously, you cannot investigate and prosecute all of them at the same time, so, like in anytng else, you do it in chronological order. If you find your backlog to be excessive, you allocate more resources.
But all of that is beside the point -- I'm not at all sure how the 'it is legal under state law, so we will not prosecute it under federal law' argument holds any water whatseoever.
And I am -astounded- that someone would accept the idea that it is OK for the President to enforce the law based on what HE thinks should and should not be illegal.
Its amazing what a (D) next to your name will get you.