A truck bomb is easier to make, and easier to get into position. A suicide bomber is more likely to get closer to the target and do more damage. There are many ways to cause mayhem, and using a light airplane is probably the most unlikely. The examples given are all nutters or suicides and none hit any target with explosives or chemicals. None of the persons so far caught by the FBI for flying in a TFR has been a terrorist and I would suggest that none ever will.
If a terrorist was flying over a building in which he knew there was a person who was his target, how does he see, or identify him(her)? And how does he successfully direct his airplane to the right spot? It looks easy on TV or movies but in the real world it is something that might be considered, then rejected in favour of several, much easier, solutions.
If the intent was to just cause fear and confusion, there is really no defence against an airplane. In the hands of a trained terrorist it would be very difficult to bring him down and the collateral damage would be greater than the damage the airplane could cause. Better to prepare for the response than to waste time and effort on trying to stop it, since the attempt would be futile. Look at London, and the response there; they admitted that stopping attacks like they went through are virtually impossible. The place Londoners thought would be their refuge (the Tube) became the killing field. Look at Bahgdad, where thousands of innocents have been killed because the checkpoints are manned by trigger-happy soldiers motivated by fear for their own lives, because of a perception that everyone is against them and every vehicle has a bomb. Do we want that in the middle of the US? Where no-one is safe from our own government? If living in fear underground does not make you safe, what are you going to do? The climate of fear and terror is fueled deliberately in the US by the government and the willing media. I always thought that the British, at least, were more pragmatic and less prone to panic. Was I wrong?
There are thousands of light airplanes flying in the US every day, and none of them is a threat, at least to a ridiculously small chance. There is a far greater chance of earthquake, fire, gas leak or accident. Spending any time being afraid of a small airplane flying overhead, much less planning to shoot one down, is paranoid to the extreme. Using my tax dollars to fuel this fantasy is insulting. That it is supported by anybody at all is frightening.