I don't think it's an "unfair battle" to compare the durability of AW and Z8 in the context of this post (use of toppers over sealants).
The person who initiated this thread was interested in a good topper to use on top of other products/sealants. However, just because we are talking about toppers doesn't mean that the issue of the durability of the topper itself is a moot point. Many of us, myself included, are interested in toppers that can provide the maximum durability - even if they are being applied over another very durable true sealant.
After all, the more durable a topper is, the longer it will be before the underlying sealant starts to break down/erode. Or to put it another way - the topper provides a "sacrificial lamb" against the elements and forces acting on/against our other underlying paint protection, and the longer the topper lasts the better for that purpose.
If AW and Z8 provide about the same durability, or if AW provides more durability than Z8 - that is one salient factor (along with many others like gloss, slickness, price, etc.) to consider in making an informed decision about which topper will best fit the needs and circumstances of each potential purchaser.