From the Zex website:
Q: What is better, a "wet" or "dry" kit?
A: It all depends on the application. A "wet" kit is ideal for both normally aspirated applications as well as forced induction applications. It can require a little bit more installation time than a "dry" kit, but is easier to tune if greater than stock HP settings are to be experimented with. A "dry" kit is excellent for normally aspirated combinations that have a return style fuel system. They are very easy to install and are a great "first time" nitrous system. It is not recommended that "dry" systems be used on forced induction engines.
By the way, Gen 3 M90'* have epoxy coating; the Gen V'* have Teflon.
In general it seems there'* mixed results about nitrious or water injection and rotor coating erosion. Some people are fine, other'* aren't...
The bottom line I gather is that dry shouldn't be used for fear of running lean under a boosted condition, and the best method for nitrous in */C applications would be direct port.