Which would you use?

No need for parking if you are cerakoating it IMO.

Parkerizing isn't that strong of a finish. It's better than bluing, but that's about it.

I'd cere-kote in the color of choice and be done.

Manganese and zinc parkerizing are available, I have found that the manganese finish such as our military uses, provides the best finish. It also works well as an undercoat for sprayed on coatings such as CeraKote and as another example the DuraCoat. I would recommed both if it will be a rough service gun, or an older gun that bluing would not suffice.
Parkerizing, or phosphate coating, has been used for over 100 years in many various forms due to its oil trapping, non-reflective characteristics when used in severe weather conditions, that is why I would recommend it as the base coating or even the only coating if need be. The finished color is dark gray but may vary based on the type and quality of the steel. Color variations can also be achieved by the use of different lubricants such as Cosmoline, linseed oil, gun oil, etc. Note too that Brass, aluminum and stainless steel may not be Parkerized.

Dictator/Cartel Gold Plating is my suggestion.

In all seriousness, I would just go with whatever is easier or costs less.

Park under another coating is a fantastic finish. Better by a good deal than either alone. Hard chrome is even better.

You can Cerakote over bare metal, but those that I trust say that a park basecoat makes for a much more durable finish.