The major difference between both the cars are engines - while the 330i is powered by a turbocharged 2.0L 4-cylinder producing 248 hp and 258 lb-ft of torque, the 340i comes with a bigger turbocharged 3.0L in-line 6 cylinders producing 320 hp and 330 lb-ft of torque. For most the buyers, the 330i xDrive would be an apt choice.