I think what you mean is that you are considering a LEAF that has a 6.6kW charger vs. the one you have that has the older and slower 3.3kW charger.
The more powerful charger will cut the charge time for your LEAF in half which is a good thing, but if you charge at night, it really does not matter.
The main issue is what year of used LEAF to consider. I would recommend that you only consider 2014 or later SV or SL trim level. These cars have a more reliable battery, the more powerful charger, and also a host of improved features like beter HVAC, suspension, charger, etc. The price delta between one of these cars and an earlier one is minimal too.
These cars are your best buy.
2014 LEAF SV
Delivery May 23 2014
46,000+ miles - all 12 bars - Same range as new - No warranty issues ever!