(Not going to go into defence, given LW and Drazinononda's treatment. For the record, I tend to build up
(especially from a plain[2]) in order to create gantries from which I can fire down, but what has just been said is entirely a justifiable reason for going largely horizontal.)I tend to aim straight down, these days, but I
have done a lot of digging into hillsides, setting up a stockade around it[1], etc. you can get (at least the first bit of) your fortress all visible on one level, for easy management.
A nice arable plain with a 'outcrop' of hills within which you can set your "doorway to the Dwarven Halls" is certainly a decent plan. Expand downwards (and out underneath the plain, especially for subterranean soil) and even upwards (not breaking back out of the hillside, unless you're also planning watchtowers) to gain extra real-estate (e.g. expand out the initial bedroom area floor-plan into the additional dimension) and you can probably work on that sort of plan for the lifetime of your fort.
Going vertical from the start has the possible benefit of finding the deeper features sooner (and rooms stacked vertically have less travel-time between them than ones spread horizontally, no matter how large each room is), but those are the roundabouts to compare with the hill-door swings, really.
Ultimately, almost every fort becomes a hybrid of many different styles, anyway.
[1] Don't forget to make your stockade line the up-hill side, if you do this, and make sure no ramps remain that let anyone path onto the top of the walls from the outside. It's annoying to be shot down upon by enemy archers, when you get this wrong.
[2] A large-
looking aboveground structure belying the far larger subterranean complex. Imagine the base in Battle Of The Planets, but in earth and stone instead of water... The "Attol" is just a convenience.