Hi,
Got your email and replied, but I'll post it here anyway, just realized you have an AOL addy, for some reason I can't send messages there
The Ace stock is lighter, it's more comfortable to shoot because the standard steel ZM stock has a rough sandpaper like surface, the Ace unit is made of aluminum, the upper tube is covered by a plastic like material so it won't freeze on your face. I don't quite like the folding mechanism though. On the ZM stock you don't have to do anything to unfold, you only have to push it down when you want to fold it. Ace stock you have to push it in an unnatural way to fold and unfold.
I talked to the owner at Ace, he said he'd put the Ace stock on the ZM mechanism, a nice guy, he used to work for ZM. He also told me they'd have new folding mechanisms soon, that was back in April :(
Also the Ace stock extension (that screws into the lower) isn't nearly long enough, when installed it only leaves a little bit inside the lower. It looks cool but if I want to fold/unfold in a hurry, and want something that will absoultely work with the ZM system (the Ace stock isn't exactly designed with ZM in mind, the ZM stock/mechanism is), Ace specifically says it's for the short recoil system. The ZM stock extension has a tube in which a synthetic buffer resides, when the bolt carrier is blown all the way back during firing, it hits the buffer instead of bare metal. There is no such accomodation with the Ace stock extension, since all it is is a piece of solid, threaded aluminum with two holes in the rear for the folding mechanism to screw in. I'd stick with the ZM stock, or send the stock and ZM folding mechanism in to have Ace install the Ace stock into the ZM mechanism.
Here's a pic:
Here's the ZM stock/folding mechanism
Here's the Ace stock/mechanism/extension