Don't take the quality of your carpet cushion lightly
The performance of many carpets will hinge on the performance of the cushion. Spending a few extra dollars on cushion can mean extra years of performance for your carpet. Some carpet warranties also hinge on the grade of cushion you select. Carpet cushion affects the luxury feel underfoot, quietness, resilience, cleaning, insulation, and, ultimately, the life of your carpet. Crushing or flattening of your carpet fiber causes it to look ugly long before it's useful wear life is over. Quality cushion will help resist this condition.
Thick and luxurious carpet can fool you at times. Determine the performance you want and the conditions effecting your carpet. A tighter twisted fiber doesn't feel as soft but it will generally give you better performance over the life of the carpet. A tightly twisted yarn will resist soiling, matting and crushing. When a yarn is not tightly twisted it may feel more luxurious but it will tend to crush and flatten out more quickly.