Only ventured to Southwest for 2 trip in the last few years. When I made the switch to Delta (from UA), it was for:
Ontime Performance
IIROPS Handling (especially when I wasn't statused)
When I need to connect SLC and DTW are GREAT!
Friendlier Staff
Seatback IFE
I am allowed a +/- $200 for work travel, so if I can get Delta in that range, its a no brainer.