The biggest difference is that anyone can say they are an “advocate.” There is no required license or certification to be a Special Education Advocate. An attorney, on the other hand, is a legal professional who is licensed to practice law in your state.
Oftentimes, advocates might be former teachers or educational specialists, so they might have expertise on the way your local school system operates. But their background and their experience can vary widely.
Special education advocates charge fees, but they are generally less expensive than attorneys (who might charge you $100 or more per hour).