The difference in Denver is so significant that United actually swapped all the Houston-based 737s that came over from Continental in the merger with the Denver

The difference in Denver is so significant that United actually swapped all the Houston-based 737s that came over from Continental in the merger with the Denver-based A320s from United. The 737 base moved to Denver and the A320 base moved to Houston. The 737 has better performance with high-altitude takeoffs.