A modern jet is stopped by brakes, ground spoilers and thrust reversers, in that order. And, the main reason for the ground spoilers is to kill the lift and increase the effectiveness of the brakes to stop the aircraft. Thrust reverse isn't even used in the calculations for stopping distances for jets, unless you are landing on a slippery runway surface. When you are stopping an aircraft, you don't really think about pollution, as safety is much more important. The engines do produce some exhaust, but it's only for a few seconds during the reverse operation. I have no idea which costs more, as again, safety is more important that cost. It costs a lot more to run off the end of a runway, so you take care of stopping the aircraft before you worry about the cost.