Huh, Interesting.
My previous ICE car was a Subaru Outback 3.6R. There were lots of things I liked about it, and it worked quite well in the snow.
The obvious pros are the drive train layout. Not all AWD systems are made equal, and Subaru has a good one. The WRX STi should also put out more power than the i3 does, and accelerate faster, though it won't have that punch of the starting line that the i3 has. The WRX is also a larger car than the i3. You could also say range, but, you probably wouldn't have bought an i3 in the first place if that was an issue.
The cons are also somewhat obvious. It's an ICE car instead of an EV. I feel like the BMW would have a little bit better luxury feel than the WRX, though you can get some nice trim packages on Subies. I also prefer BMW's electronics (iDrive) to Subaru's system. The ride feels a little bit more refined on my BMW. I feel like my i3 takes corners a little bit better, but I had an Outback, which has worse handling than a WRX STi.
I assume you have a proper winter/snow tire package for your i3? If not, you should really get one if you live where there is heavy snow, it might change your mind about the car's winter performance. You can also get tire chains for severe weather. That being said, AWD cars typically do better than RWD cars in the snow.