I find that the best way for restructuring the landscape is to use ramps. If you channel, then miners have the tendency to channel the tiles from under eachother. If you dig ramps, then they all work on the same level and have no such issues. (well, less ) As channels remove the floor and the rock below, ramps remove the rock being dug and the floor above.
While i find them to be a lot more convenient and safe than channels, ramps also have a few pitfalls:
1) Trees - when digging ramps under a tree, the floor tile the tree rests on cannot be removed. When you ramp below and around such a tree, the floor tile will cave in, potentially causing massive injuries. Always clear the area of trees and make sure none grow on the work site while you are digging.
2) Lack of support: Ramps provide no support for adjacent tiles. Since you relying on the ramps to remove the ceiling above, in cases where there is nothing to dig (say a previously dug tunnel) the floor tiles above will not be removed. You have to remove these orphaned floor tiles BEFORE you ramp around them or they will fall. It is rather easy to overlook and miss them, especially if you are working on large and complex constructions.
These 2 reasons are why I prefer channeling to ramping
when working on the surface. Both methods work for strip mining, ramping tends to be more dangerous for me. Especially when removing surface tiles, trees tend to pop up while removing large areas. Ramping will ignore the tree, channeling stops because you can't channel a tree.
Removing 1 level at a time means if your dwarves channel under each other, they only fall 1 level. Special care needs to be taken when digging over the top of a pit.
Floor tiles without walls directly under them should be removed first, whichever way you dig. Channeling and ramping will both leave those floors unsupported, causing cave-ins and possibly dwarven deaths.
When working underground, I tend to dig pits by designating ramps. This way I don't accidentally channel out the floor over the cavern, or something equally stupid. Just watch out for removing the bottom of murky pools, or the layer of stone under an aquifer. Channeling won't give you any warnings about warm or damp stone below the level the channel is designated on, and ramping won't warn you about the level above.
JMZ