***6 Blade Water Pump vs 8 Blade??***

Ok they say a 6 Blade pump is for Iron Heads and an 8 Blade is for Aluminum heads.

Can someone explain why?

I am going to buy a mechanical pump for my Twin Turbo Build which will mostly be a street car and I am leaning toward the 8 Blade. I am running the RHS Indy X Heads.