Here's some math for you. In 10 years, UAL will retire roughly 6,000 pilots just due to Age 65, or roughly 45% of their pilot group. In the same timeframe, SWA will retire just under 3400 pilots due to Age 65, which is currently roughly 37% of the pilot group.
At UAL, you'll be able to upgrade at the 90-some percent system seniority because you'll have other options. You'll still be way, way junior in the company, but you'll have the opportunity to upgrade simply due to fleet diversity. At UAL, it appears that the retirements alone will make your upward movement dizzyingly fast, not to mention the expansion.
At SWA, you'll upgrade to 737 captain upon reaching 60% system seniority. You'll have to have roughly 4-5,000 people below you. By the time you reach the junior most captain position at SWA, by comparison you should be able to hold 756 captain at UAL, having been a NB captain there for years and years. Your upgrade will have to come from mostly growth and though SWA will grow, having a single fleet type and being a mature airline are seriously the two biggest impediments to your upgrade.
No telling what the future holds, but given the simple math, if I had multiple CJO's and were starting from the bottom, I'd go to UAL and not look back.