I prefer the upper deck - but will often go downstairs to walk around. It is a bit pokey. Business class in the "nose" is also OK - there is no through traffic.

It is nice having the lockers by the window seats. But, they seem to be quite narrow. Also the overhead lockers are shallower than downstairs. The upshot of all this is that if you have something particularly bulky you might need to store it in a cupboard, whereas downstairs you might still be able to fit it in the locker.

For EVA air pax in C, look out for flights where no F class is sold but they run a 747 with F in the nose. This is certainly preferable to C upstairs. They will normally happily allocate you to an F seat on request at check-in. (Might do this without asking.) On one TPE/SGN flight I had F in the nose to myself whilst several people were upstairs. Food service was C though (as expected).