I'd pony up the extra money and go for the FoolProof-unless you never intend a future sight adjustment. If you plan on sighting your rifle in and never changing it, the 5D will suit your purposes and save you money to boot. If you ever decide to change your sight adjustment to accomodate different bullet weights, distances, etc., the micro-adjustment FoolProof receiver sight makes good sense.
As Caliper_RWVA noted, Williams are good people.