Compare the number of inches of recline and the width of the seat. Some airlines offer adjustable lumbar support, leg rests or head rests.
Compare the food and beverage service and flight attendant support. Some airlines offer meals with economy plus, and the cabin service is friendlier.
Price is a huge issue when comparing economy with premium economy seats. Seat Guru, which runs a comparison chart on citing the amenities of several airlines' premium economy cabin, recommends paying no more than 10\% to 15\% more than standard economy for premium economy. The website suggests that premium economy seats are not worth more.
"Premium economy" is a fare class designed to give travelers a bit more luxury without the steep price jump to business class or first class. Usually premium economy offers slightly larger seats and further recline, but exact options vary by airline.