Evan I have experience with both so I will try and give you some info here.Oil Spray
I had my pickup truck oiled every year for about 10 years since it was new. Everything seemed to be good until I started having to repair stuff. What a mess. I used to drive a lot of dirt roads and what happened was the dirt would layer up with the oil. The next year you get sprayed the oil can't penetrate the previous layer so it keeps building up year after year. Moisture gets trapped between the layers and the metal and rusts underneath without you noticing it until it is too late. Not so much body panels as the dirt usually doesn't get in there but all the frame and body attachment items are what get layered and rust. I eventually had to jack up my truck and go at it with a pressure washer for about 4 hours.
Bottom line on oil spray is if you don't do too much backroad driving you should be ok. No drip is the way to go. Still before you get a reapplication you should go through a car wash that has the underbody spray.Electronic Rust control
The theory of this is a long known process used in the water and waste water industry of using sacrificial electrodes. There are two kinds. Passive and active.
Passive uses a buried electrode which is attached to a watermain or tank and its molecules replace the molecules lost by a rusting watermain or tank. When something rusts a small electric current is created and rust is created as certain charged molecules dissapate.
Active impresses a current flow in the opposite direction through the sacrificial electrodes to prevent your vehicle body panels from losing their molecules.
My neighbour had an early version of this on his pickup years ago. It was used for fuel tank transporting and had small dents and scratches all over it. I never saw any rust on it. My camper van has an auxillary battery on board that has a common ground with the starting battery that I keep a trickle charger on it when it is parked. It is an 88 Dodge and the only place it has rust is where the body panels and frame attachments are somewhat insulated from the rest of the vehicle and it doesn't amount to much on a 20 year old van. How many old Doge vans can you find with virtually no rust on them??? Chargers work on the same principal of reversing the flow of electrons to re-energize the battery plates so this is kind of the same. My wife's car (2004 Impalla) has the unit you are consisdering or a similar one. It has two sacrificial electrodes attached to the front fenders. She hit a raised curb about two months ago (GRRRRRRR
) and scratced both passenger doors near the rocker panel. As we all know we have had almost as much rain in Ontario as Noah faced but their is not a speck of rust showing yet.
The better units have sacrificial electrodes installed on all four corners of the vehical but as long as you have a good attachment point and the transmitter is working properly then the two point attachment should be sufficient for most cars. If you see your electrodes pitting away you know you have a problem that needs to be addressed somewhere on your car.
Bottom line for me is I am going with the rust modules, use a good quality car wash a few times a year and buy the spray cans from Crown or Oil Guard for known trouble spots. Oil is messy. Especially if you need to work on your car.