SWA has a relatively young pilot group, with IIRC a relatively linear age curve. Ie, your seniority will progress slowly.
UAL (like AA and DL) has a lot of retirements coming up. Plus a wider variety of flying on bigger planes, more opportunity for better, and varied, schedules and QOL. Your seniority progression will lead to more pay.
Much as I like SWA, all else being equal I think UAL is a clear choice, especially since you're older. You might never upgrade at SWA.
5 yr retirements for majors, FDX and UPS.