Don't forget it's not just extra kWh, it's also really nice suspension & brakes, including ABS on the '15. It was a no-brainer for me to get the '15.
The powertank is included on the 2014 only, if I want it on the 2015 its about 3000$ extra.
How much better are the suspension on the 2015? Im a heavy rider, so that could me rather important to me...
I figure that the Showa suspension is worth about $1000 USD, over the Fast Ace suspension. It will provide a smoother and more controlled ride, especially when cornering around bumpy back road corners. In my opinion the Fast Ace suspension seems to work better the more weight it is carrying and the front fork is certainly strong enough for the weight of the bike. I have my suspension settings dialed down as low as they will go, but I did up my rear shock preload a couple of turns from stock. I would say that the rear shock spring is a little weak.
I weigh about 160 pounds, dressed for riding. My suggestion is to test ride the 2014 model and see how you like the suspension. If you like it, the power tank option is really worthwhile, as it gives a noticeable increase in range, compared with the 11.2 S. I also think the extra 40 pounds located where the fuel tank on an IC bike would be provides a better balance and puts more weight on the front wheel. But the 2015 bike does have ABS, which is worth about $500 USD and might make the bike easier to resell, even if you don't feel the need for ABS.
I would say that if you really value the power tank, get the 2014, but if you don't need the extra range, then the 2015 model might be the better deal for you. decisions, decisions.