We've gone to the Soul EV side and after a few hundred miles, we're glad we have.
First off, we really liked our Leaf and we seriously considered leasing a 2015 but after looking at the Soul EV Plus, we leased it instead. It's clear that Kia did its research and created a car to compete directly against the Leaf. We had the Leaf SL so we're comparing top-of-the-line to top-of-the-line.
What do we like about the Soul compared to the '12 Leaf (no particular order and not comprehensive - just off the top of my head)?
Range is better
Mileage gauge is more accurate
Automatic folding mirrors
B mode (regenerative braking)
Heated and cooled seats (front)
Heated seats (rear)
Navigation is better
Bluetooth quality is better
HD Radio included along with XM and Pandora
Adjustable steering response (normal, comfort, sport)
Leather seating (not an option in '12 for the Leaf)
6.6 charging (again for the '12, 3.3)
Chademo is standard
Cruise control, actually all steering controls, are better quality
Steering wheel has blue stitching
Nicer feel to the radio controls on computer interface
Better power train warranty (if we decide to buy it at the end of the lease)
Because of the earlier EV infrastructure limitations, we put a mere 14k over three years on the Leaf (not the Leaf's fault certainly)
Lease is less expensive (to be fair, we were early Leaf adopters and ordered ours from the factory and paid full bore - never again!)
How about the Leaf?
The Leaf is definitely faster
We liked that the Leaf looked unusual; the Soul looks like every other hamster Soul out there
The Leaf wheels are nicer (not fans of the Soul EV wheels)
We prefer the Cayenne Red color (Soul EV comes in white, blue and black)
Navigation updates appear to be over the air(?) - the Soul EV needs an updated SIM