United flies widebodies as domestic only aircraft, which gives them more widebody seats (they don't have as many more Captains as the numbers imply due to work rules differences though).

United has some good work rules, but I think Delta still has the better overall contractual package and is heads and tails better for a commuter (at Delta you can reserve a jumpseat in advance and it's yours, at United you can be bumped at any time by someone more senior).

The retirement numbers are high enough at either for you to have a great career going forward. I do think the financials are better at Delta, but don't think either airline is a bad choice.

Right now new hires are getting put in NYC, but within the past year SLC has been in new hire classes at Delta with any base in the system attainable within a year (longest time now is having to wait for the 1 year new hire seat lock if you need to change equipment to get the base you want, normal seat lock otherwise is 2 years).

Upgrade isn't nearly as critical at the major level as at the regionals, if I understand correctly that you're talking about moving to wherever you can upgrade fastest base wise (for Delta the fastest upgrade is almost assuredly going to be NYC FWIW).

UAL does provide new hire hotels and I believe new hire pay (before you get line qualified) is better. Delta provides hotels during indoc but once you come back for initial aircraft specific training you're on your own (hotels are provided again for recurrent going forward).