I am so very mixed in my building style. Really, I base my ideas off of the terrain I find and the order that I find it.
For instance, in one of my single-player maps, I built a big house on a natural island just off the coast of the mainland. The island stretches up from water level to be at least thirty blocks high, and that's where I built a house. I then proceeded to hollow out the island and furnish the interior with all manner of things. I also built a single mineshaft that has served me well - diamond without much searching at all.
Conversely, in the multiplayer server I have with my friend, I built an expansive castle system built into the side of a cliff. I built a large system of walls around a space, filled the interior courtyard with buildings and even built a gigantic keep. I still have room to build more things, as well!
Shortly after I finished most of the buildings within, though, my buddy suggested we make a tram system between our bases of operation (he built an island in the middle of some water and then built a fortress on top of that). I agreed, and we thought it would be cool if we did it entirely underground. So we did. Of course, seeing as his base is on an island, this involved having to make a stretch of track underwater. So we did that, and I helped him build an underwater room attached to his base for the tram to exit out into. (By "help" I mean "did it for him".)
So yeah. I guess, to be fair, most of my construction is above water level, but it does involve excavating and hollowing mountains out. I have tried my hand at underground building too, though, and am presently fiddling with a massive underground construction project. So yeah.
Mixed.