Fictional, but the main thing is that it is of high quality and high effort.

Both. As long as they're realistic (flight performance and stuff) and not unreasonably futuristic and overkill.

i like fictional builds more since it's more about new creative work
but it's easy to fall into common design traps like not allocating enough space for stuff, pancake plane syndrome, unfeasible aero or construction, etc

Both, but I love fictional more.

Fictional