Your Leaf S+QC has the faster 6.6 kW (6.0 actual to the battery) on board charger, so for maximum charging speed you will need an EVSE that can deliver 30 or more amps (6.6 kW / 240 volts = 27.5 amps).
Note though that if you will be using it in different outlets, getting the largest EVSE you can afford might limit your abilities to plug in on the road, because you can't plug a 30 amp EVSE into a circuit that can only handle 16 or 20 amps.
Since you are looking at a portable charging solution, seriously consider upgrading the stock Nissan EVSE to dual voltage operation by sending it and about $300 to EVSE Upgrade. http://evseupgrade.com/?main_page=product_info&products_id=2" onclick="window.open(this.href);return false;
Phil the owner is on this board under the username Ingineer and many Leafers have upgraded theirs through him. Check in the Marketplace section as Leafers facing the end of their leases often advertise them for sale with a part-exchange for an unmodified unit to give back with the car.
For the 2013+ North American Leafs, the EVSE-Upgraded OEM EVSE can be switched to draw either 16 or 20 amps, to give you charging flexibility. 20 amps won't charge your Leaf at the max speed, but unless the battery is very low you may not see much of a difference anyway.
If you want a "manufactured" dual-voltage solution, AeroVironment (the same company that makes the "official" charging station for Nissan USA) has a product called the TurboCord. The dual-voltage version is over twice as expensive as the EVSE upgrade ($649) and only supplies a maximum of 16 amps.