How much is it to buy a ticket?
Apparently for the specific flights you want, UA has no saver space available. Instead, you are finding redemption at 2x the saver level. It's basically last seat availability. That's a very dear redemption. I only redeem at the saver level. It depends on how cash rich you are, and whether you have other opportunities for the miles. This will help you determine whether to spend cash or miles.
If I read you correctly, and I might not, you can spend:
50K - 2 Southwest tickets
100K - 2 UA tickets
I would choose the Southwest redemption UNLESS travel time on UA is several hours shorter.