I ponder this question almost every time I'm about to "get out".
If I'm carrying more than a 30 pound pack, I generally opt for boots. It may be habit -- maybe an old belief system, -- but I feel more "surefooted" in boots under a heavy load.
If I'm going to be in deep snow, I prefer to be in ski boots rather than Invernos.
However, I have carried ski boots more frequently the past few years in the local Mountains (if skinning was likely to be a pain) and instead hiked up in a treated leather boot. (Think Baldy.)
If I'm going to be on a lot of scree, I bring lightweight gaiters regardless. And if there's going to be some scrambling, the shoes Dave Daley recommended a while back are great, 5.10 Camp Fours.