Look at it this way:
-Globalist, neo con, who-ever-else conspiracy:
very hight tech (missiles, drones, tons of explosives rigged secretely, voice-imitating technology, passengers kidnapped, etc), millions of dollars and alot, A LOT of people involved in secrecy.
-the Al Quaida conspiracy:
very low-tech (cutter knives, pilot training, physical training, good scheduling, secretive internet communication), rather inexpensive (living costs, passports), very few people involved and a lot, A LOT of anger towards the USA.
Now tell me which one is more credible.