I did my first transatlantics on DC8s and 707s. The first trips had no inflight entertainment. A year or two later there was one film shown on a series of screens pulled down from the roof over the aisle. Comfort? Probably better than today - the seat pitch in economy was generally better than today.
Then the 747 and DC10 arrived and the comfort began to go downhill (slowly to start with) but, if you want IFE it began to improve - you got multiple films!
The arrival of the 767 on long haul was something I took a while to get used to. I was offered the opportunity to fly London/St Louis/Kansas City and chose to go via Chicago (747) instead. Once I understood how the enhanced maintenance and flight planning worked I overcame my reluctance and flew 767s and later 777s. What I find interesting is that some of the enhancements introduced for long haul twins found their way on to some quads.
The 757 is one of my favouite aircraft. Given half a chance they take off like a homesick Angel. They are stable and comfortable (well, comfort is also about the airline fit so that's definitely not as good as when I started).
United? I am not a fan of any US carriers. They are safe, but bland would be kind. Have you seen the Emirates advert with Jennifer Anniston? I'm afraid the fun they poke at an airline (assumed to be a US airline) during her dream is not that far from the truth in my experience.
United have regular economy and economy plus. Plus is either chargeable or offered to their frequent flyers or used to fill up with you and me if they need to. It's only a seat with more legroom but if you can score plus then do so.