Depends on what you want to use the car for.
15's are small valve heads, 6X's are large valve head although they have a larger combustion chamber than the 15's thus lower comp ratio.
The line in the ad "been sitting for years ready to use" would worry me. Best to look them over REAL good! But then again, who knows what kind of hack job some unknown machine shop or Joe Blow did to those 15's. If it were me I'd pass on them......my .02
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