I Have a Mopar Performance electronic control (P4120534) and would like to test it and see if it still works. I've used an (MSD-6) many years ago and was wondering which is a better control?....Thanks

A local parts store will offer an ecu testing service. Likely at no charge.

As for which is better? Well, lots of pro's and con's to each. I'll only say I like a stock style perf ecu, because if it strands you, stock parts are easily obtained to get you rolling again.

So, is the MP unit near stock?

P4120534 is an upgraded stock ignition system. The way it works is to amplify the impulses from the Hall effect diodes in the distributor to determine when to fire each plug. Because there is no friction like the old points had and because the heavy current switching is done by the shiny heat sinked transistor, it is extremely reliable for long periods of time. My stock ignition has 170,000 miles and is 35 years old and holds timing extremely well.
The MSD ignition brings a lot more things to the table. It is in the class of "capacitive discharge" ignitions. It still needs that timing pulse from the distributor but it converts 12 volts to a higher voltage, charges a capacitor to that higher voltage and discharges it multiple times after the timing pulse arrives. This gives you a much "hotter" spark than stock. It results in plugs seemingly lasting longer as it can fire a pretty fouled plug. It results in cleaner and more complete combustion. They can both be used together without any problems.
As for testing, the P4120534 either works or it does not. There really is no in between. And ECM testers do not work on this part.
Hope this helps.
Craig

I have an MSD distributor already, so could the MSD-6 control be used with my MSD distributor? And would it give a more complete burn than the P4120534 control that I have now?

Yes, an MSD ignition box will work with an MSD distributor.

It will give you the multiple spark up to 3000 rpm and will give a single strong spark well past 8000 rpm.

You can always use one of these...I no longer need it if someone wants to add it to their collection.

Here's the other side of it.