Let me correct you on one factual error, before replying to the meat: The Spark has a 21.4 kwH battery, of which 17.3 kWh is usable; the i3 has a 22 kWh battery, 18.8 kWh of which is usable.
The i3 is a somewhat larger car than the Spark, although considerably smaller than a LEAF (see the post...