Neither DL nor UA or their partners will upgrade you for free to business on long-haul international as that is not a status benefit. It can happen rarely when coach is full and flights are oversold and they need to make room, but it's not something you can actually count on (they won't upgrade you just because there are open seats up front). DL Skyteam AF/KL partners also won't upgrade you on their short haul flights because they don't offer comp upgrades to their own elites.
Once you hit you Gold status on Delta, you will have Skyteam ElitePlus status and will be able to enter Sky Clubs and Skyteam partner lounges on international travel (except for Caribbean flights). If you are into card churn and manufactured spend to earn points for long haul international, then UA is definitely your airline as it's points are more valuable for international travel and Amex doesn't really allow much in the way of card churning.
Last edited by xliioper; Jan 14, 2022 at 12:36 pm