I just made a coffee-infused red IPA; I added about 2.5 oz of medium-roast ground beans to the carboy after fermentation had tailed off. It gives a firm but not overwhelming coffee taste, though of course that's a significantly lighter beer than a porter.
You should think about the methods as falling into two broad categories. Either the coffee beans are in hot liquid, or they're in cold liquid. Just as with brewing normal coffee, the hot methods will give you more acidity and harsh flavors, while the cold methods will require more coffee and will give you rounder, less harsh flavors. It's the same reason that you might want to cold steep your dark grains.
I don't have experience with potion vs cold-brewed additions vs direct steeping in the carboy, but I doubt it makes a big difference--they're all cold steeping. The nice thing about using a potion or a cold-brewed addition at bottling is that you can better control the balance of coffee and beer--try it with a sample, see what you like, then scale up.
As for the lightness or darkness of the roast, think about the flavor profile of the beer itself. For a dark beer like a porter, I could imagine pretty dark coffee flavors working well. But you might also think that *really* dark, espresso or French roast kinds of flavors would clash--maybe that's more for stout than for porter. Your call. Then again, if you're adding vanilla, the beer will be pretty forgiving.