Diamond Certified is an independent rating company. It verifies the contractor, business or an entity credentials such Contractor standing and status, number, Workers Comp., Liability Insurance and any other accreditation such Lead Safe Remodeler. They conduct an annual phone survey of random customers and ask a set of question and record customer testimonials. They also mediate any dispute between the contractor and the consumer.
Buildzoom actually verifies insurance, license, etc. They even verify your BBB rating and whether or not you are an accredited business with them. I have had a circumstance myself where they actually followed up with the customer to evaluate their experience. From my experiences this has been done yearly as well. The only added benefit I see to the Diamond Certified is the mediation of dispute… Of course once obtaining accreditation with the BBB that is already done.