Two very distinct mechanisms, which most likely evolved independently, are employed for ATP-driven H+ pumps: the rotary mechanism of the V-ATPase and the alternating access mechanism used by the P-ATPases (Fig 1). The significantly more complex V-ATPase consists of 25–39 protein chains compared to a monomeric or homodimeric polypeptide for the P-ATPase. The operating mechanism for the V-ATPase is also more elaborate consisting of an electric motor-like rotary mechanism. In contrast, the P-ATPase operates by switching between two (E1 and E2) conformations similar to most allosteric mechanisms.How do we measure "significantly more complex" in terms of bits?
Mechanism is discussed in more detail here.