I haven't seen the Ace in action at all but I'll try to give you coverstock differences since I'm fairly familiar with Accu-Tread.
IMHO, the Xception will be much stronger downlane because of a) the GOO cover is stronger than Accu-Tread and b) it's pure resin. Accu-Tread for me was cleaner and a little weaker downlane than PowerPlus/SuperFlex. GOO is stronger than PowerPlus. I don't know how strong the core is in the Ace, but the core in the Xception is extremly strong. So, given that the Xception has a stronger cover and at least as strong a core, I would assume that the Xception would be stronger overall...especially downlane.
Hope that helps.
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Anthony Chapman