Advantage: Southwest...
No change fees
One-way tickets
ESPECIALLY one-way freebies
No advance requirements on freebies

Advantage: United...*
An assigned seat
E+
International freebies
A shot at First or Business, especially international

Beyond this, it's even: It's a coach seat on a full plane, or it doesn't matter.

Interestingly, as I fly UA transcon out of BOS very frequently, where there's a lot of competition for upgrades, I am looking at E+ seats a lot (particularly an exit row, as on Southwest). In my observation, it doesn't look like UA is selling the E+ any more than Southwest is selling BS -- e.g., "The Gap" for Southwest equals a lot of empty E+ seats for UA. Unfortunately for UA, per se, they don't get to fill those seats like Southwest does (backfilling). I still wonder if these programs will last in their current forms, and I'm not just stigmatizing Southwest.

By the way, I would move the bar that nsx sets to include 1P on UA, and maybe 2P (25K), except for the no-change-fee.

EDIT:
*Plus, not mentioned before, I don't think: On reaching 1P, three transcon, or East Coast-DEN, or West Coast-ORD roundtrips yield ~25K -- a freebie.