As appropriate to the situation…

Also both simple and complex can easiliy mean different things:

Simple can describe a few rooms (eg a Cape Cod or Saltbox) or a simple layout with many floors and rooms (many typical 'glass-box' office and apartment buildings where each floor is a copy of the others).
It can also refer to a lack of decoration or (in MC) to the absence of RS devices.

For MC purposes, complex can also mean a house that encompasses several functions (eg enchanting room, anvil room, fishing hole, auto-cooker/smelter, portal room, storage system with sorter, and various manual- and auto-farms).
[For an extreme IRL example of this see https://en.wikipedia.org/wiki/House_on_the_Rock ]

Im gonna say complex because this is a complex answer ;)

It's kind of difficult to choose.
I think it depends on the style you're going for. A typical american residential house (as an example) is supposed to look a bit simple instead of extremly complex, while something like a hyper-modern skyscraper is deftly best as a more complex build than simplified :p