The Fair Tax is easier to evade. Either a dishonest buyer or seller can evade the Fair Tax. Dishonest Buyer: Presents a sales tax exemption certificate for items of personal consumption. Dishonest Seller: Collects sales tax but fails to remit it.
There are checks on exemption certificates and on sellers, inventory vs sales, just like IRS does now.
A dishonest seller can evade the income tax by simply not filing. A dishonest buyer cannot evade the income tax.
I see how you completely avoided answering how may collection points there are for each system. obviously 80% fewer collection points will be easier to police under a Fair Tax. There are no buyers and sellers under the income tax, but for a buyer to avoid the sales tax it would take either a cooperating seller (conspiracy) or as you suggested a fraudulent exemption certificate...far harder to avoid under a Fair Tax.
You are caught up in static economic analysis. A change in the tax code will cause people to change behavior. Many people will create business out of their hobbies to acquire a sales tax exemption certificate.
Dynamic economic analysis suggest the number of collection points will increase as well and the number of sales tax exemption certificates. Both developments will increase the amount of fraud.