It's what is currently on Amazon

The 32 amp charger would fully maximize the on-board chargers of the i3, and the 40 amp wouldn't offer any faster charging on the i3.

However, the price difference is usually negligible, and purchasing a 40 amp charger would give you room for growth in the future for an EV with faster on-board chargers.

Additionally a 40amp charger would be properly spec'd for your 50 amp breaker, as you're supposed to operate at 80% breaker capacity (50 * .8 = 40 amps)

I have the JuiceBox Pro 40 and have been happy with it.