having owned both 325is and 318is. the 325 was a little more power.. mostly torque (not that you need to go fast in the winter), both cars hold 140+ very well on the highway, both are good on gas, both have about the same insurance rate, both are fun to drive!
But the 318 might give you some problems because the motor takes so long to heat up and warm the cab, and mine didn't like to start when it's extreamly cold.
I have a few e30 parts for sale, updated often, discounts for local pickup/delivery. Check out my eBay
account for details.