I’ve flown on the 777 a few times. Depending on the configuration, it’s not really as comfortable as any other jet not even in first class. Sleeper seats are like a hard padded futon.
I have no idea the difference between a 777 and 777x - I am guessing software. But maybe like the 737Max they moved the engines and reinforced the wings.
Did fly a 787 once. Relatively short distance for what it is capable of. Nicer.
The difference is basically the wing. The 777x wing is significantly larger than the 777.
I have flown on a 777 several times. Very, very nice flight. No complaints. Have not flown on a 787...yet.
Full disclosure: 32-year Boeing (retired) electrical engineer.