Can't comment on United (well, not until I go in November) but I just got back to the UK from the USA. We flew with KLM/Delta. Out of the 5 flights, I think just one was actually a Delta crew/Delta run.
My flight from Amsterdam-Chicago with KLM about a fortnight ago was one of the best that I have ever had. Despite booking the tickets ages ago altogether, my 3 year-old-son, infant in arms, husband and myself were not seated together. I phoned and asked but was told nothing could be done. KLM flight attendants on-board were incredibly helpful. In the end, it was a very nice passenger who obliged so we could be together but the staff the rest of the flight kept coming round asking if our kids needed anything, making faces to entertain the kids, etc., making it super tolerable.
Same things both times Manchester-Amsterdam, KLM run-super nice staff. Also able to check-in online with an infant ticket (not sure if that's an issue with you.) By contrast, not able to with Delta and then had a problem when I went to their partner KLM flight, as Delta hadn't inputted something into the computer correctly.
Every Delta transatlantic flight that we've been on I've found the flight attendants to be really sarcastic and pretty much rude to most people. I've not found any of their online/checking in things particularly convenient and the planes seem to be older and not have the onflight entertainment you sort of expect these days (no individual tv screens?Really??!!)
Sorry-I just flew and am jetlagged and up, so typing away!
But I got there (mostly) on-time or early and safe, so good in that respect!