how much difference is there between 502 mag heads and hp500s both carb motors looking to sell my 500 heads to a freing but want to make sure there will be a gain for him.

If they are both Gen VI heads, then they should be the same casting (088, I think). The 500 heads will have rocker studs and roller rockers and the 502 Mag heads have a non-adjustble valvetrain with stamped steel rockers.
Eddie

Valve size the same? I thought the HP500's had 2.25 vs 2.19?

I think that is correct as well as what Eddie said.

thanks guys the castings are the same but the valves are bigger and they are a better valve.

I don't believe there would be a noticeable enough gain to warrant the change. Let alone the expense of new head gaskets.