By standard joint, I assume he meant saw cut or possibly grooved control joints? A keyed joint is usually created with a stay-in-place metal strip with with a built in key. The cut or grooved joint creates a weak spot for the concrete to crack and depends on aggregate interlock to keep the crack from displacing vertically. The metal joint physically separates the concrete and uses the key to prevent displacement. The purpose of both is to avoid unsightly random cracks in favor of nice straight cracks.
I'd be surprised if he will install the keyed joints for the same cost as saw cut joints but even if he did, I wouldn't mess with them. There is too much chance the metal form will get moved during placement resulting in a crooked joint. They often come with a plastic cap to prevent rust but I'm not thrilled with having metal a few millimeters from the surface of my concrete. Finally, if you are using a vapor barrier, the metal stakes that support the key create dozens of holes that need to be patched.
Saw cut joints are not without their pitfalls but they are the industry standard and are used on millions of square feet of slabs. Hope that helps and good luck with your slab placement.
