plus, if you want to make Captain pretty much right away, Denver has unfilled 737 and 320 Captain spots.

With UALs various fleets in DEN and TK there's tons more flexibility for your career than just flying the line in the 737. UAL is probably the better long term choice.

yes agree, I posted it

Observations:
Many a United pilots hired in the last 2 years came from WN. Some never look back, some occasionally mention "well back at Southwest we di _____" the use of the We pronoun indicates a mild level of regret...I think that will go away when they are on the Sydney overnights.
To my knowledge ZERO pilots hired at SWA in the last two years came from United.

I was hired at WN about a year and half ago. I had a guy in my class from UA who lived in Houston. He didn’t like the guys he flew with at UA and that was his primary reason for leaving.

If you’re over about 45-48, I’d consider sticking it out with SWA. Below that, your QoL from seniority at UAL will overtake SWA within 10-15 years. Both are solid and SWA has initial advantage, but over time the options in DEN hold more promise. And UAL 737 training will be a breeze.

I can’t speak to what life is like at SWA, so the flexibility of our schedule is all I know. All things come with seniority, here. The more junior you are the less flexibility you have. I haven’t worked more than 10 days in a month in years at this point, but it took me years and some changes to our trading system to get there. PTWOB… funny, haven’t heard that term in years.

Simliar situation for me last year, left SkyWest for SWA and decided to update the 'current employer' section of my apps in the middle of SWA training.. United and Delta both rang within a week, after about 6 months of crickets back at Skywheezy being fresh in the left seat.

I'm local to the Bay Area so I had OAK/SFO to choose from depending on the airline, similiar to having your pick of airline in DEN.. For me it came down to what would give me the most time at home and the least gray hair over the next 30 yrs.. My last flight with SWA was my IOE sign-off trip, currently home-based on the WB side with United.. I've been averaging about 1 trip every 5-6 weeks. Granted that is apples to oranges in comparison to the NB side.

Being able to choose your flavor of flying with United, plus the benefits of DEN expansion and the Training Center I think make it an easy decision if you plan on staying in the area long term. I had a fantastic experience going through training with SWA, and all my classmate buddies over there coming off probation right now are all happy campers.. I would happily have stayed if I didnt get the call from UAL. You have the benefit of choice between home-base airlines, hard to go wrong.

A few things to consider... both airlines hired thousands of pilots in the last several years.

Do you think you'll see those SYD layovers? As a captain? Or would it be better to have flexibility to make your own schedule and go play overseas on your own time and your own dime because it's easier to throttle your schedule?

On the other hand, Southwest is a grind. It's a domestic airline pilot grunt work with much nicer Hawaii trips than the legacies because we don't do redeyes from there, unless redeyes are your thing, in which case it's United hands down.

It really boils down to what you want, what your priorities are, then roll the dice and you'll find out if you made the right decision the day you retire.

I had a very similar situation. Denver based, less than an hour from the airport. Skywest previously, got hired by SWA, updated my app with SWA as my new employer and within a week had an interview offer from UAL. I started new hire training and was happy enough at SWA and passed on the interview with UAL.
Fast forward 9 months, I was tired of the SWA schedule and started wondering if I could fly that way as an old guy. I resubmitted my UAL app and long story short am now working at UAL.
For me personally, the thought of waiting to upgrade seven-ish years at SWA, working the wild AM or PM schedule and having no other options than to fly the guppy pushed me over. I have been happy at UAL, especially being in the Denver area. I was also working on the old SWA contract so of course things have gotten better over there recently.
No wrong answer, just depends on what you want.