Thanks for starting this thread, I need to ask the same question! In my case I started traveling extensively for work. Thanks to a AMEX bonus offer, I am gold medallion on my way to platinum and expect to be diamond by the end of the year with Delta. The only problem is that it looks like I'm moving to Chicago! Am I crazy to think that I should continue on Delta????? I am in Milwaukee and already have to connect through Detroit for most of my flights, and I find that I do get upgraded quite quickly. In addition, being the only loyal Delta Chicago-based flyer should help me continue with those free domestic upgrades, right?

More info on my flying-90% for business, no one place in particular, but Atlanta does come up once a month. 90% in coach. All domestic...not sure if that helps?