The bus or taxi trip to Abu Simbel from Aswan takes three or four hours. All buses and taxis leave at the same time, escorted by several police cars. The air trip takes half the time, but it also costs much more.
If you travel to Egypt in Summer, you better choose an air conditioned bus. The trip from Aswan to Abu Simbel is not a problem, because most of the travel takes place by night and first thing in the morning. But the trip back to Aswan can be a nightmare in an overcrowded bus without air conditioned. Don't save your money here, or you will regret afterwards ...