Most wet nitrous kits that I have had experience with are very similar, Nitrous Express (NX) and Nitrous Oxide Systems (NOS).Both of them have nitrous and fuel solenoids, a fuel pump, a throttle position switch, arming switch, the option of either using the WOT switch or a push button to activate the system and a lot of wiring.
The good thing about the Nitrous Express and Nitrous Oxide System kits is that you have some very good tech support with them.When you want to jet up or down for HP both kits have some pretty straightforward charts as to what fuel and nitrous jets to use so that you don't run lean, rich or exceed the flow of the fuel pump that comes with the kit.
One thing that I have noticed about Nitrous Express kits is that they seem to underrate their horsepower numbers on their systems for cars.Several of my friends have made more power at the rear wheels on the dyno than the Nitrous Express kit was rated for.
I don't have any experience with dry nitrous kits for ATV's, only cars, so I really couldn't give a good opinion or advice on those kits for ATV's.All of the kits that I have ever used or my friends have used seem to be very reliable as long as you stick to the guidelines that come with them and don't go too crazy with how big of a shot that you are using.That seems to be where most people start running into problems.
Here's a picture of the Nitrous Oxide Systems kit that I run on my Polaris Sport.
The dry NOS, Such as the Boon Docker kit, pressurizes the float bowl through the venting system, this allows more fuel to be added when nitrous is being used. Getting that system adjusted right is very important.
Boon has good directions and a memometer for this set up. I would use a A/F meter with any of the NOS kits that you go with.
With a dry system, you inject the gas typically instead of the liquid. Dry systems are typically above the carb while pressurizing the float bowls to put extra fuel into the cylinders. Wet systems are usually below the carbs (not always) and use an additional fuel cicuit to fatten things up.
The dry is usually (not always) easier to setup and safer for the motor.