DAL has dramatically outperformed AAL over the past 15 months or so since AAL topped out. DAL has dramatically less debt than AAL. The market always considers debt levels/balance sheet condition. Along with the many other considerations necessary to make a good investment, you'd be wise to consider debt levels also.
If you're going to use P/E, back out the debt when you calculate it. AAL has approximately $25 per share in net debt which means the market is valuing AAL at approximately $57 per share net of debt. With earnings forecast to come in at $5.70, P/E is +/- 10. Comparatively, DAL real P/E comes in +/- 8.