HI,
Most of these big shot immgration attorneys charge an arm and a leg for filing NIW or perm or j1 waivers. You will mostly be dealing with paralegals or junior attoneys.Most of work is done by paralegals filling your forms and you will be responsible for supplying all the supporting documents. I know from friend's experience that some attorney's dont even know when to file EB-2 vs EB-3 which can make a big difference.
Go with an attorney who responds promptly to you emails, phones call and not crazy busy (where your file is the last one in stack).