Maybe if you are only comparing the diff / rg ratings they might appear close.  The Eraser won't be able to handle the same amount of oil as the ACE.  The Ace should have a stronger midlane to it and a harder flip on the backend.  My *guess* is atleast 5-8 boards difference if not more.
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-Chris: DJ's Pro Shop : Auburn, MA

only in the core numbers, the accu-tread coverstock is much more aggressive than the Pro-thane the PBT is wrapped in. You will find the coverstock is the most deciding factor in the hooking ability of a ball. The Accu-tread cover is Storm's 2nd most agressive coverstock, providing much more gripping power whereas Pro-thane is one of the least. As stated the Ace will handle more oil and longer oil than the PBT. Just the nature of the beast.