What is the difference between the Audi A6 45 TFSI and 55 TFSI?

I am considering buying an Audi A6 and I am confused about the different engine options. What is the difference between the Audi A6 45 TFSI and 55 TFSI? How does it affect the performance and fuel efficiency?

The main difference between the two engine options is the size and power. The A6 45 TFSI has a 2.0-liter four-cylinder turbocharged engine with 248 horsepower and 273 lb.-ft. of torque. On the other hand, the A6 55 TFSI has a 3.0-liter V6 turbocharged engine with 335 horsepower and 369 lb.-ft. of torque.