It depends. There are a couple of people here that owned a Leaf previously. The two eco modes can save power, but they come with some conditions. For the most part, if you're very conscientious, you can achieve nearly similar economy by just being more careful and selecting your options. The eco modes dampen the accelerator pedal so you'll accelerate slower with the same input verses comfort mode. It also minimizes the climate control operations. This forces you to accelerate slower unless you really floor it. Some people like driving with lots of clothes on in the winter, and essentially turn the heat off (the car will detect the windows fogging, and turn it on, regardless, up to a point).
Second thing is whether you can regularly set a departure time. This preconditions the battery, and you have the option of warming the cabin. Done while connected to the EVSE, you'll maximize range (you can precondition the cabin without being hooked to the EVSE, but not the battery pack). As my handle indicates, I live in Nashua, NH and have had my i3 through last winter. The vast majority of my use is for shorter distances (the reason I bought it), so I did not really worry about maximum range or setting a departure time. I've seen the computer suggest a max range of more than 90-miles in the summer, but I've also seen it as low as 50-miles or so in the winter. The range estimate is based on your last 18-miles of travel along with some current battery temperature and voltage level, so, depending on the trip you have planned, may not represent your real capacity. For example, since most of my trips are short and in the winter, the heat may have to crank up to max for them, it is not anywhere near representative of someone driving 20-30 miles to work. I've driven as much as 10-miles and not had the estimated range to empty change (and once in awhile, it could go up!) because of the type of driving I'm currently doing verses what I had been doing.
Your worst hit is lots of short trips where the car can cool off in between and you crank up the heat. The BEV is more efficient than the REx because the BEV has a heat pump verses the REx only having resistance heating. If it is super cold, the heat pump isn't very efficient, so even the BEV may need to use its resistance heating, and the difference with the REx is not as great. The REx adds weight, so your range verses the BEV is lower, but then, you'll have the REx to back you up. My use pattern wasn't worth the extra money, maintenance, and complexities to justify adding the REx. Your use pattern may want it.
IMHO, the i3 is more efficient than the Leaf in what it does. And, it's a lot more fun to drive.
There are lots of threads where people talk about how the battery range changes in the cold. While you may not need it, if you select a destination and use the nav system, it takes into account the terrain, the speed limits, and the current traffic conditions, and the range estimate will be more accurate.
