About 6 months ago I was faced with same option. I am 39 now.
I took the first class date and I am happy- the company is probably the most solid of the big 3 legacy carriers. They have other ventures like credit card, engine maitenience and fuel refinery- so all the eggs are not in one basket. They seem to be expanding slowly but surely with both WB and NB aircraft. I guess being invested in other international carriers is good for the company, but bad for wide body aspirations of pilots, but I can't do anything about that except invest and push ALPA for more scope on that. The bet on airbus over boeing payed off at least for now.
In general I am treated very well. I am west coast, and I prefer stay away from ATL as much as possible. Like any big base your relative seniority cannot be understated when it comes to QOL- you might want re-think only living in IAH or ATL. Moving for better QOL should always be on the table.
My best guess is 5-10 years to upgrade on NB depending on how picky I am with the base. It might be sooner. If I want to go right seat on WB it will be longer for the 350 and shorter for the 330 or 765. I might make WB captain in my last 3-7 years with no seniority in the seat- I guess I will figure that part out in about 20 years- no sense worrying about it now.
Reserve rules are really good- but like anywhere else- commuting to short call is not fun.
I hope the hiring continues at close to 1k a year so I can get pushed up from the bottom. Given the general health and diet choices of about half of the boomers I sit next to with I think the retirements might be a bit faster as they lose their medicals a lot earlier than 65.
The goal not talked about on these forums enough is to base your lifestyle on year 2-3 NB FO income and not have any ex-wives. If I am not happy at 200-250k annually then I am doing it wrong because that should be way more than enough
United is a good place to be as well I would think. They have lots of orders. Boeing is also sending ZERO 737s out right now. Up to 6 (realistically 4) 787s are coming out of South Carolina a month with a 750 unit backlog. How many and how fast UA gets their order is a wild card. Whether or not they can fill the seats of 250 787s is a wildcard (maybe we will going to london for 99 dollars a seat) Financially I think they are slightly below delta- but I am not an analyst so no opinion is really valid with that. I just know UA is dependent on massive growth.....maybe it happens and maybe it doesn't. Maybe you get a class date and sit around for months or years getting paid min guarantee because they have more pilots than planes. Maybe economy slows and they have more seats available than pax that want to buy them. Or maybe they get 250 WBs and you get left seat in under 10 years and ride it out for the next 20 making a million bucks a year flying 10 days a month to exotic locations with the perfect family and get a giant mansion with a ferrari and a yacht.
Either company you choose- I think you will be fine. You will know if you made the right decision 30 years from now. Or an astroid might hit us tomorrow and it won't matter. Good luck