Also, fyi, the B300 can be charged with 700W solar input. So it can be a stand alone DC power source with solar charging capability if you want to take it camping or whatever. Compare that to the EF slave batteries and how much space they take up and without the versatility.
This first photo shows the relative size of two B300s next to an AC200Max, which is the same size as an AC200P
Compare the sleek lines of the EP500Pro and imagine a couple B300s neatly stacked, to the Delta Pro with its slave batteries that can only be charged when connected to the Delta Pro, and nothing can be stacked.
They should not be left outside as shown, either.
I found this Q&A in the comments on the EF campaign page.