Well, they each contain the same amount of energy, as 12V∗400Ah=24V∗200Ah=4800Wh=4.8kWh .
The voltage of the batteries is different and therefore they’re not simply interchangable, as device are designed to operate off a certain voltage. Applying 24V to a device designed to operate on 12V would likely fry that device.
If you’re designing from scratch, the 24V system will be a bit more efficient — all else being equal, since you will have less power loss in the wiring ( P=I2R ) because of the higher voltage, which means you can reduce the current by half ( P=VI , thus if V is doubled then I is halved for the same amount of power). Concretely, since I is halved, the power loss in the wiring will be 4 times lower — or your wiring could be cheaper. In relatively high-power applications, this can be significant.
There are many other factors in deciding whether to opt for 12V or 24V . 12V is more common and thus more devices that work off it can be readily obtained. On the other hand, many household devices like shavers, laptops etc. work in the range of 5 – 20VDC , which means that you can simply step down the voltage from a 24V battery to power these devices with a simpler and more efficient — albeit only slightly — buck converter rather than a buck-boost converter to get to the same range from a 12V battery.
In practice, unless you have a good reason not to, it’s best to stick with a 12V system simply because it’s so common. It’s good to stick to conventions unless you have a good reason not to.
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