Len,
I wouldn't limit it to those two choices, but believe there's a whole spectrum of possibilities between the two alternatives you propose.
Also, I think you're overestimating the level of effort required to place the charges. The charges aren't dynamite, they're thermite, which simply needs to be placed on the surface of the metal, not drilled into the metal. All the building gutting you discuss is primarily related to removing the hazardous materials prior to a professional implosion, to prevent the dispersion of asbestos, lead, silica, and other potential respiratory hazards that need to be controlled during a planned demolition.
Don't get me wrong, it still seams incredulous to suggest that someone could do that without being detected. But on the other hand, if I really believed that buildings could collapse the way they did on 9/11 from a fire...or from a relatively mild seismic disturbance, I would be too terrified to ever step foot in a city again.
"Our houses are such unwieldy property that we are often imprisoned rather than housed in them." - Henry David Thoreau, Walden: Economy, 1854