My 2015 came with the OEM 120 volt unit and a "Primecom" adjustable unit that has choices up to 32 amps. Given that 24 amps @240 is around 5.75 Kw so 32 amps is not needed and 24 amps can be hooked to a 30 amp 240 amp circuit. However when fed 208 volts it take more than 27 amps for the same output.
So for most users, who will not be connecting to two legs of a three phase power source (208 volts), a 30 amp 240 volt supply would more or less max out the 6 Kw onboard charger.
A 50 amp branch circuit takes 6 ga cable from the breaker-box, more copper, more money. It would be worth it if you are a dedicated EV user and plan to buy newer vehicle with a bigger on board charger than the Leaf takes. A 30 amp circuit can be supplied with 10 ga (if the run is short enough) or at most 8 ga. Wire is cheaper and the conduit is smaller (where required).
A note about circuit rating, a 30 amp can provide 30 amps for a momentary surge, but will trip if you try an hold that 30 amps for a long period, so the 80% rule kicks in, and 24 amps is the safe continuous load. 50 amp can handle 40 amps continuous. 40 amps, 32 continuous.
I ran 50 amps because both the wire and conduit came free to me, and allows future expansion if I ever get a higher charger car.