Go to United.
Relative seniority at United, even if identical to SWA, would allow you to have better overall seniority in NB seats. With retirements + growth, you'll have more options at United. SWA isn't meeting their hiring goals and it's going to slow down any planned fleet growth. Hell, they're having major problems rebuilding their pre-COVID network. Everyone is dealing with mini meltdowns, but SWA's operation is in for turbulence right now.