I have been facing the same decision and for me it comes down to living in domicile. Moving to a Delta base is not an option and I live in a senior Southwest base so I will probably stay at Southwest, however, if I lived within 2-3hrs of a junior Delta base like NYC it wouldn't be a question, I would leave. Their retirement crushes ours. A Captain who I flew with and helps manage our 401k ran the numbers for me and it was close to a 1.5-2 million dollar difference for someone starting in this industry in their early 30's. Our retirement needs some serious work.