The better apples to apples in regards to retirements is the percentage of gummers hitting 65 and when. I've seen here that 50% will be 65 at dal in 2020 or 2021 and the same 50% at sw in 2028-30. Of course this is assuming no organic growth (mergers don't count) or shrinkage.You either have fuselage envy............ or you don't. Flying the 737 for "the rest of my career" doesn't bother me. I actually really enjoy being 3 time zones max from home if my family needs me.
Either would be a good choice..... today. I'd look at future retirements as a major player. WN will retire about 140-150/yr. indefinitely. I think DAL has many more.
Gup