6mm head bolt engine vs 8mm head bolt engine?

I have not tore into one of these engines yet but I have 2 of them and the one I'm running right now is a straight plug version with the 6mm head bolts, and the spare sitting on the bench is an angle plug engine with the 8mm head bolts, just wondering if there are any differenced in these engines other than the angled plug and the head bolt size.
I am thinking that maybe the 6mm engines is just a lighter duty engine on the top but basically the same other than that, anyone know the facts on this?
Thanks, Shan