From your question, I'm guessing you are debating because you can get a really good deal on the 2014 but then you are not sure if this car is perfect for you so you wanna lease the 2015 and find out?
In my opinion it depends on the incentives ($$$$), if both Federal & State are available then I would do the 2 years lease and take the advantage of the incentives every 2 years no questions asked.
I would also lease if this is my first EV, to make sure this is the right car for me, with every weather conditions all year long. And if my Federal tax liberty is far less than $7500, then I couldn't take the full benefit of the Federal zero emission credit. If that's the case then I would lease it, the leasing company could get the full credit for me to reduce the leasing rate.
If I know that car is perfect for me and would like to keep it for the next 5~10 years, then I would try to negotiate for the older model for the better price. Since one can typically get the 2015 i3 for 15~20% off MSRP, I would try something like ~23%. If that doesn't work than I can still go back to the leasing option.
Don't worry about i3 will depreciate fast because others are making 200 miles range EVs. Tesla already did, and the cheapest one costs $70,000 plus tax. I'm certain others would just be as expensive as well. Think of it this way...you can wait for the iPhone 6S to come out to buy the iPhone 6 for a little discount, but don't expect it to drop more than 30%.
As for the warranty and maintenance....theoretically, EV is very low maintenance on pretty much everything except the battery.
With minimum fluids and mechanical parts, it shouldn't break down easily but then again it's just "theoretically." It really depends on BMW's QC on this model and only time can tell.