While the sand in the purge button and soft cover with a DPV issues have been covered well in the posts above, there is one more reason the Zenith is my clear choice over the D1.
There is a small design flaw in the D1 which has been corrected in the Zenith, that is only noticeable on servicing/adjustment: The barrel holding the poppet and lever is not locked into position like it is on a Scubapro for example, by a square end. Instead, it slides into position and is tightened with an external jam nut. When a hose is attached to the reg, it is possible to rotate the entire barrel by ~3-5 degrees from vertical. This is easily seen before you put the cover on and corrected. However, if a hose is replaced with the cover on, or a twist is left uncorrected, one leg of the lever will be higher than the other, creating increased friction at the diaphragm pad as well as possibly engaging with the diaphragm earlier than it should. It might even occur during normal dive use when a pressurized hose is stiff, if the jam nut is not tight enough.
When I was taking the service seminar from a HOG dealer, I noticed this problem during the tuning phase and I was told this has been corrected in the Zenith (though I haven't taken that version apart yet).
