NEW YORK ( TheStreet) -- One of the first stories I wrote in Atlanta was about Delta Air Lines' ( DAL) founder, C.E. Woolman. When current CEO Richard Anderson gave Woolman a shout-out at the start of the air safety video on a recent trip, I knew the company had changed for the better. LUV), having grown the top line 50% since 2009 and made a small profit for the last three years. Anderson has steadily paid down debt so it's now a little more than 25% of assets, and since the recession ended, the company has been a cash-flow machine, generating $9.5 billion in operating cash flow from 2009-2012, and more than $3 billion more in just the last two quarters. Airlines are generally considered to be among the worst investments you can make. They're down there with auto companies. Since the start of the year Delta stock is up 83%, but even so, it has a price-to-earnings ratio of just 10.63, which is positively Ford ( F)-like. If its earnings were valued like Southwest's, which has a P/E of more than 25, this would be a $50 stock. The Southwest-Delta battle now shapes up as one of the more interesting in American business, and Southwest retains some key advantages. Airfleets.net said last year that Delta's fleet was almost five years older than Southwest's, which flies only 737s. Delta planes averaged nearly 17 years of age, with nine different types of planes from three different makers. Southwest's back-and-forth route structure remains more economical than Delta's hub-and-spoke system. It has much better fuel efficiency, which Delta started addressing this month through an order of 40 new planes from Airbus. AviationPros.com, its market cap is twice as large as Southwest's and its balance sheet is coming into line.