Wet systems really allow more adjustability through the nitrous system itself.
Its harder to get a correct a/f with a dry setup since the only way to adjust your fuel is through fuel pressure unless you have a full standalone system that can be wired to recognize a WOT switch and can change maps on the fly. You CAN just use fuel pressure to get your target a/f, and this is fine for power on the bottle, but makes off bottle conditions rich and less desireable.
With a wet shot, you can set up your pill to be at a target a/f through the nitrous setup and not sacrifice any off-bottle driveability and power loss.
Theres really no difference in power potential between the two, but wet systems are generally safer. Be careful with a wet sytem though. If you run a hand switch to activate the system, you can damage the motor if you hit the rev limiter or mis-shift. When you hit the limter, it cuts fuel, but your nitrous keeps sraying, whcih can be deadly to your motor, but also can cause massive backfires with the extra fuel being introduced through the jets and the spark getting cut.
-josh