They are both 502's
So your question is what's the difference between the 500HP and the 500 EFI.
The only issue I know of on the 500HP is change the valve springs at 200 hours
Thought of another one , if the 500HP has the gil exhaust and the risers are low you have some risk of reversion.
Oh one other thing, I believe that the 500HP was blueprinted and balanced, the 500 EFI isn't.