Here is my story and while it might not exactly address your questions, it does give food for thought. I have nearly 1M on both DL and AA as over the past ten years I have split my travel almost 50/50 between the two airlines and always based out of BNA. This is based on my work also in the IT industry.

For almost five years, I was heavily traveling to LAX, DFW, ORD, and LGA and AA became a no-brainer due to directs to all. Prior to that (and currently) I travel to every part of the US, with an emphasis on (stay with me sports fans) Big-12, SEC and ACC country. And that has meant DL for me. While WN has the most destinations, coverage isnt good enough to make a huge difference and the departure times always seem odd to me for business purposes.

With all that said, I have never been very price sensitive, prefer mainline equipment that accommodates upgrades, and for the most part do not mind reasonable connections. Keep in mind that I fly alot, not as much as many on this board but qualify for PM on segments by mid-year and mileage by OCT, so I find obtaining status not really a problem.

Lastly I have found that with the NWA addition, DTW and MEM has opened up some improved routing options for my needs (who knows if they will last). And with SLC services returning, the west coast options will improve.