I've been doing a lot of window-shopping and research into the 4th gen Ram 2500/3500 series, so I find myself asking the same question.
I think getting the dual 220amp alternators is the way to go if you plan on overlanding with your truck (or at least getting the 180amp + 220amp setup). Between winches, lights, air compressors, fridges, it would make sense having the extra amperage. If you read the stats for some of the heavier duty winches out there, their amp draw can easily match or exceed the max 440amp capacity that the Ram can come with.
Not that you're always going to be winching the vehicle and drawing those kinds of electrical loads, but still I'd rather have the extra capacity and not need it versus the other way around. You could potentially buy a truck with a single 180 or 220amp alternator and upgrade to duals later; I don't know how costly or complicated such an upgrade would be.
I don't see a downside to getting the truck from the factory with the extra alternator already installed. The extra alternator shouldn't be reducing fuel economy unless you are actually using it to power extra devices. Go with the duals, if you can find a truck that has them or order it from the factory like that....my 2 cents.
Edit: Also it's spelled Cummins not Cummings.